Using the rules on perceiving and identifying a spell being cast here: https://www.dndbeyond.com/sources/xgte/dungeon-masters-tools#Spellcasting
If someone were to be attempting to avoid scrutiny while casting a spell using the Ready Action (which indicates that the spell is cast and then HELD for the Reaction phase if actually used) would you have them make an Ability Check to hide the casting?
The spell in question (Dispel Magic) has both Verbal and Somatic components.

What sort of check?
Arcana? Sleight of Hand?  Deception?

And opposed by what?
Perception? Investigation?  Active or Passive?

The PC is being actively watched and is supposed to be 'surrendering'

To my mind, a spell being prepared as a readied action is not cast and then held, but rather just one that they are prepared to cast at a moments notice - any material components are in hand, and they are ready to say the words and wave their hands as required.

I would view a subtly cast magic as involving sleight of hand for the somatic component, as you will be trying to do so subtly, and probably deception or even stealth for the verbal part. The goal is to say the words quietly without moving your lips, and to wave your hands without appearing to do so. Sleight of hand is probably the best catch-all for this, as you are basically trying to do something without someone noticing.

As they are being actively watched, I would have it as a rolled perception check for the captors to notice. If they were simply nearby and making sure the PC doesn't go walking off, then I would use passive perception.

I think there is a lot of misconception on the Ready Action:
"Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn."  It doesn't let you hold it until an opportune moment... it lets you use an action to cast all but the last bit on your turn, and then use a reaction before the start of your next turn to trigger it.  Situationally, this looks like it would just use up the spell slot with no advantage over just casting the spell.

Now, if the caster is a Sorcerer, they have a perfectly valid way to do this.  The Subtle Spell metamagic option allows a spell to be cast without any verbal or somatic components.  If they are not, this simple option will not help them.

After that... good luck.  Trying to hide the Verbal component can be challenging: "Most spells require the chanting of mystic words. The words themselves aren't the source of the spell's power; rather, the particular combination of sounds, with specific pitch and resonance, sets the threads of magic in motion."  The mention of specific pitch and resonance are particularly troubling for a spellcaster that is being observed.  To me, this is the hardest one to disguise.  It's not really about Stealth or Deception, because they are having to focus on the spell.  Maybe Arcana at Disadvantage to hope you can say what you need to say and the guards watching you are not able to hear what you are saying or see your lips moving.  Or throw something like Performance into the mix (if they aren't a Charisma based caster).  Or, use their spellcasting stat as a straight roll at disadvantage.  Yeah, that might be what I would do...

Also, the Somatic component needs some care: "Spellcasting gestures might include a forceful gesticulation or an intricate set of gestures. If a spell requires a somatic component, the caster must have free use of at least one hand to perform these gestures."  A foreceful motion or an intricate series of motions are going to be easily detectable unless they can hide them.  I would say that Sleight of Hand would be the way to go here if that was the only component involved.

When you have to do Material, Somatic and Verbal to cast a spell while being watched?  That's a tough call.  Definitely at Disadvantage.  If the other was passive perception, then this should be rolled for and there might be a bonus to the guards as they are ready for some kind of shenanigans (like cover rules).

 After that... see Xalthu's answer below.  I like it much better than mine...

I don't think that I'd allow it to work at all. Subtle Spell is a metamagic ability that a sorcerer needs to choose as a class option -- forgoing some other option, and they only get four such choices in their whole career -- and then spend sorcery points to activate. Letting someone do it with only a skill check is giving it away for free to a member of another class (or to a sorc who chose a different metamagic). I'm not saying this because I'm worried about offending the sorcerer in the party, but because giving away class features for free is a bad idea generally.

Quote from Xalthu >>

I don't think that I'd allow it to work at all. Subtle Spell is a metamagic ability that a sorcerer needs to choose as a class option - 

Personally, I don't feel like Subtle Spell and trying to hide your spellcasting are the same thing. 
Subtle Spell actually removes any need for the Verbal and Somatic components while casting that spell.  You don't have to struggle to keep your gestures hidden or your voice quiet, or be skilled with Deception or Sleight of Hand or the like. You spend a Sorcery Point and could potentially cast a spell while hogtied and gagged.  Well, arguments could be made about whether they have to have a Material Component (or Focus) in their hand or not...
IMO, that's well worth the purchase of the class option.  
For purposes of this discussion, that removes 2 of the 3 aspects which might allow for a spell to be perceived during casting.  

No subtle spell casting, although that blanket ruling reflex is now wondering about Arcane Tricksters maybe having something up their sleeve.

What I'd recommend the PC do is review the components of their spell list.  A few, like Misty Step with a simple verbal, are almost designed for this situation.  One of my parties has a standard operation procedure when they apprehend suspected spell casters where they bind, blind and gag them because of past escapes from their custody that had bad repercussions.

No surprise lighting bolts.

I have been letting players do this for years, a simple slight of hand check to cover the hand movement and muttering of the spell words, I will not allow it for spells that are more Obvious (you cant slight of hand a fireball), or that require involved use of physical components. 

Bless has the same components as fireball, sure when the fireball goes off everyone knows it has been cast but there is no indication that the casting of it is more obvious.

Verbal components require a particular pitch and resonance except for message which specifically says only the target can hear I rule that the volume is about normal speach volume.

Somanatic components "might include a forceful gesticulation or an intricate set of gestures" but I am not aware of any indication of which spells have intricate gestures and which have forceful. I tend to think of this as something as noceable as putting your hand in the air or doing a thumbs up.

If the caster isn't being particularly watched I might allow a stealth roll to see if they can perform the components without anyone noticing for example if the party are in a town and the wizard casts major image of the local God telling the townsfolk to do a particular action.  However if the party is being watched there is no way a spell can be cast without being noticed unless the caster is using subtle casting.  This includes circumstances where the observing it not suspecting of a spell about to be cast, go into a bar say "there is an in wind in the air" and cast prestidigitation to create a puff of wind the bartender will know you cast a spell (maybe an int check to know it was what caused the wind).

I wouldn't allow this. Xalthu's post above is a good reason why. Ultimately, spells have these various components to balance this sort of thing.

There are some edge cases, such as if there is a lot of commotion, crowds, distance, or the target wasn't aware of the caster, in which I might allow a stealth check - but it's all situational. I think I remember a Sage Advice tweet suggesting the same thing.

But the situation the OP describes wouldn't fit that requirement at my table. Perhaps I don't have all the information, but from what you've described, there's been a battle, in the player's current round, they've agreed to surrender, but are actually wanting to take the 'ready action' to cast a spell. What is the trigger? If they're actively being watched, there's just no way in my mind to do this on the sly.

Quote from Jegpeg >>

Bless has the same components as fireball,

 Except that the first sentence of fireball says "A bright streak flashes from your pointing finger to a point you choose within range and then blossoms..." so makes it kind-of impossible to cast that subtly.

Bless would simply require you to hold your holy symbol, which is even easier if the symbol is on the front of your shield that you're already holding.

If you Ready the spell, you definitely need to fulfill all of its components to ready the spell, but it is not clear whether you need to fulfill any of them to 'release with your reaction'. If you don't, it is possible to Ready a spell while in a location where you cannot be seen, then walk to a location where you can see, and release the spell, and the casting would not be visible, though if the spell has a visible manifestation (most evocations and conjurations do) that part will be visible.

The rules don't specifically address using sleight of hand, but given that using a skill is normally an action, to use it to conceal spellcasting you either need a bonus action spell, or the ability to perform sleight of hand as a bonus action (or a special ability that is specific to the situation, such as the arcane trickster's Mage Hand Legerdemain).

When you Ready a spell, you have already cast that spell. If the trigger doesn't come up before your next turn, you lose the spell slot (if it has one). It is expended; no ifs, ands, or buts. If the spell requires a material component, then you've already manipulated that. If the spell requires a gesture and/or incantation, then you've already performed them. There's no real hiding that.

In the scenario you describe where the PC is surrendering, I would assume they are being watched very closely. I would have them make both the Slight of Hand for the Somatic and a Deception for the Verbal components and they would need to make both checks. I would also say it would be an active check on the part of the NPCs. It's a tense situation and they would be paying a lot of attention to make sure that an actual surrender is happening. I also might have them NPCs do perception for the verbal component (do they hear the spell casting under the PCs breath) but Investigation for the Sleight of Hand because it's a question of whether they put together that the hand movements are potentially threatening.

To cast a spell without it being noticed, you need to be a sorcerer using Metamagic, or under another magical effect.

Remember that all action within a sequence of Turns is happening simultaneously, and the Turns are just a tool to make combat manageable. Really, all dice from a single Turn would be rolling at the same time. Therefore if you're trying to claim abilities functioning differently around Readied Actions, you're applying a Turn sequence to the theatre of the mind, and the two do not match up.

Ready Action on a spell really just means that you've cast your spell, waiting to release it within the next 6 seconds. Therefore you still have to say the magical words, you still have to make the gestures, and it still happens basically at the same time that you cast it, only held by a matter of, at most, 5 seconds.

When making rules decisions as a DM, always ask yourself this: Am I allowing something that mirrors or is very similar to a class ability as something that all players now have access to? If the answer is YES, then disallow it.

An example is this: My Barbarian player has a chain forged to his battleaxe handle, which links to a manacle around his wrist. Congratulations, for the cost of about 5gp he can no longer ever be fully disarmed! But wait... although this makes sense, preventing Disarms is a feature for the Eldritch Knight class... and I've just given it to the barbarian for 5gp. How then do I argue to my EK that he can't Rage because he's really really angry? Technically rage isn't magical...

Quote from Sanvael >>

An example is this: My Barbarian player has a chain forged to his battleaxe handle, which links to a manacle around his wrist. Congratulations, for the cost of about 5gp he can no longer ever be fully disarmed! But wait... although this makes sense, preventing Disarms is a feature for the Eldritch Knight class... and I've just given it to the barbarian for 5gp. How then do I argue to my EK that he can't Rage because he's really really angry? Technically rage isn't magical...

 Ouch. If the guy ever does disarmed then he's going to have an axe swinging into his leg, instead of clattering to the floor. So he might get tripped by the axe, or take damage, and then still have to retrieve it from where it is swinging wildly into a comfortable hand position...

The ideal way to do this is via Subtle Spell or similar effects.  No check needed.

I would allow a wand activation to be hidden by a sleight of hand check.  DC 13 if you have sleeves, DC 15 if you do not.

I would allow an opposed Deception check vs Arcana skill to hide a Verbal component.  Penalty to your deception of -1 per spell level.

I would a Sleight of Hand check at DC 10 + spell level to hide a Somantic OR a Material component.

I would a Sleight of Hand check at DC 15 + spell level to hide a Somantic AND a Material component.

I would

Actively watched after just surrendering? This is one of those cases where if I allowed a roll, the DC would be so high it wouldn't matter. So it would just be a flat no.

Now maybe if the whole party got in on it and wanted to create a distraction or something, I could start to warm up to that idea.

If the watchers let you get completely out of sight and hearing, sure, you can ready a spell while there and cast it when you come out, but there's no good reason the watchers would permit that.

1. While still out of sight, ready spell with appropriate trigger.
2. Walk out and surrender.
3. Profit.