If subtle spell metamagic is used on a spell with no material components, how noticeable is it? Can observes tell that a spell was cast? Can observers tell that a spell came from the sorcerer? Or is this a DM's call on a spell by spell basis?

Just for some examples,

Blindness / Deafness (Verbal components only). Does the target know that the sorcerer did it? Can a third party notice the target being affected by magic?

Burning hands (Verbal and Somatic only). Is it obvious that the fire is originating from the sorcerer?

Meteor Swam (Verbal and Somatic only). Is it noticeable that the sorcerer cast it?

If the spell has no material component and Subtle spell is used then it is virtually impossible to notice or detect the spellcasting - which also means it cannot be counterspelled and nobody will have any way of knowing who cast the spell (they might suspect the sorc if they know the sorc is a sorc with such an ability).

Basically, if there's no M component and subtle spell is used then all anyone is going to see is the sorc just standing there, no movement, no hand-waving, no muttering of words, nothing, just a person standing/sitting/lying there doing nothing (or doing something normal like drinking ale or eating an apple, or reading a book, etc).

HOWEVER

It is going to depend on the spell. A spell that directly emanates from the caster like a firebolt or affects the caster like Mirror Image is going to be noticed as coming from the sorc. A spell that doesn't detail this - like Hold Person, Meteor Swarm, etc, will have the effect just happen out of nowhere. The clue will be in the spell. Burning Hands is detailed as coming from the casters hands - that will still be the case. So Burning Hands would be noticeable. Meteor Swarm doesn't detail it comes from the caster, the meteors just appear in the air and smash the targeted areas - this will not be noticeable as being cast by the sorc.

S and V components are separate from spell text - and are magic gestures and words. If a sorc used subtle spell they won't need to utter magic words or make magic gestures with their hands - but they'll still have to "hold their hands with thumbs touching and fingers spread" as a "thin sheet of flames shoots forth from their outstretched fingertips" as this is in the spell text. So that'll be noticeable. If they used Eyebite their eyes still turn black, that will be noticeable. Spells that don't list these things, like Hold Person, won't be noticeable as there's nothing to notice (I mean other than the target just suddenly stop moving).

That being said, by the time the "spell text" comes into play, be it noticeable or not, the spell has been cast and it's too late to counterspell, so Subtle spell still retains that benefit.