Does this have to be cast as a Ritual? 

No it doesn't have to be cast as a ritual. You can expend a spell slot to cast is as an action or you can spend an extra 10 minutes casting it to use it without expending a spell slot.

But you can cast it as a ritual, meaning you can spend 10 minutes to cast but don’t expend a spell slot.

Can detect magic locate a fiend or celestial who is inhabiting the body of a mortal?

Ritual casting adds 10 minutes to cast time.

That would most likely be a Detect Good and Evil or a Divine sense Action. One of my players is possessed only the paladin who used the Detect Good and Evil spell knows.

But, if it's cast as a ritual, it doesn't expend a spell slot, does it?

There are many comments arguing or sharing what colors correspond with which school of magic so I thought I would give my two cents. I simply based it off of the school icons on D&D Beyond.

Abjuration - Cyan

Conjuration - Yellow

Divination - Dark Blue

Enchantment - Hot Pink

Evocation - Dark Red

Illusion - Light Purple/Lavender

Necromancy - Mint Green

Transmutation - Orange

Quote from RedHood5229 >>

Does this have to be cast as a Ritual? 

 Gkhhu

So, fun fact that I've never seen anyone remember in game: You only detect the presence of magic with this spell, not the direction, silhouette, or school of the magical aura, until you use an action to do so.
Fun fact number two people forget about: You can not see invisible creatures with this spell, because the creature must be visible to have its magic be detected.

So, fun fact that I've never seen anyone remember in game: You only detect the presence of magic with this spell, not the direction, silhouette, or school of the magical aura, until you use an action to do so.
Fun fact number two people forget about: You can not see invisible creatures with this spell, because the creature must be visible to have its magic be detected.

Can I ask where you found that out? I am reading the spell and based on the way it is written, the caster would only detect that there is magic nearby as the result of an invisible creature (assuming it is invisible as a result of a spell or other MAGICAL effect) but they would not know the direction or school because there is no visible target to use your action on. Or is this incorrect based on some ruling? This is pretty much how I am going to rule it going forward since I am the GM and all that, unless there is a source that I can find.

Quote from novadragon07 >>

 

 

 

Beyond 20

 

 

 

 

 Quote from Jety_Lefr >>

So, fun fact that I've never seen anyone remember in game: You only detect the presence of magic with this spell, not the direction, silhouette, or school of the magical aura, until you use an action to do so.
Fun fact number two people forget about: You can not see invisible creatures with this spell, because the creature must be visible to have its magic be detected.

 

Can I ask where you found that out? I am reading the spell and based on the way it is written, the caster would only detect that there is magic nearby as the result of an invisible creature (assuming it is invisible as a result of a spell or other MAGICAL effect) but they would not know the direction or school because there is no visible target to use your action on. Or is this incorrect based on some ruling? This is pretty much how I am going to rule it going forward since I am the GM and all that, unless there is a source that I can find.

No, your ruling is right, but it doesn't conflict with Jety_Lefr's quote anyway. Say my Firbolg, Winter, uses her racial feature to cast Detect Magic, and an assassin is hidden ten feet behind her, invisible thanks to the Invisibility spell. When Winter casts Detect Magic, she'll learn that there is magic present within 30 feet of her, but she does not know that it is 10 feet behind her, only that it's somewhere in that 30 foot radius, and she also does not know what school of magic it is, and cannot use an action to know either of those, since said assassin hidden by illusion magic is invisible to her. Jety's facts conflict with neither RAW nor your ruling, since that's exactly how Detect Magic is supposed to work.

Quote from taven03 >>

Also Detect Magic does not let you see invisible objects but does let you know that something magical is in the area aka 5 foot square?

thus you would get disadvantage but you would know something is in that area

 
Bit of a necro but answering just in case someone else reads this and wonders the same nowadays.. You would sense magic within 30' of you, not in a specific square. This is how it normally works too, then you use your action to focus, if lets say a magic sword was lying on the ground then it would show up with a magic aura around it once you focused, the invisible creature would give no results though. Same with a hidden trap, even if magic. You would sense something magic is near, just not where or what. 

Wait wouldn’t the colors indicating the different detected magics be the ones used for the symbols of the school. Like blue for abjuration, red for evocation, green for necromancy, pink for enchantment, gold for conjuration, e.t.c.?

Possibly a strange question, but does anyone know how the page source is determined for this spell?
> Basic Rules , pg. 231
This is the indicated source. However, in the 5e SRD the spell is listed on page 134 while in the 5e Basic Rules PDF the spell is on page 91. Where does 231 come from? Is there a paper book that has drastically smaller (and thus many more) pages than the PDF?

Would detect magic potentially detect someone having the dream spell used to invade their dreams?

Quote from WoopyDoop >>

So, can someone see through a magic illusion that I cast if they cast detect magic? Making sure so that I can play an illusionist properly. Also because some guy in my group says that he’ll be able to see through my illusions 100% of the time because of some High-Elf detect magic cantrip, and plots to ruin the illusions. 

Any illusions will have an illusion aura to it. That doesn't let the player see through it, but should clue them in something is amiss. It doesn't tell them if the object is an illusion, or a different thing masked by an illusion like disguise self, or a object with some other illusory magic, like magic mouth or dimulacrum.

Also this isn't a cantrip so you can't get it from high-elf abilities.

Only those that can do rituals can do this though

The [R] icon is bad UX.  No information should be conveyed only by an icon, and the icon is too small to read easily.  Casting Time should say "1 action (or 10 minutes as a ritual)".  Duration should also say "Concentration".  The <C> and [R] icons don't even have tooltips.  There is text in the class tag but it's not tool tipping.

There is no bug report option so you get a comment.

Quote from Tennyson360 >>

The [R] icon is bad UX.  No information should be conveyed only by an icon, and the icon is too small to read easily.  Casting Time should say "1 action (or 10 minutes as a ritual)".  Duration should also say "Concentration".  The <C> and [R] icons don't even have tooltips.  There is text in the class tag but it's not tool tipping.

There is no bug report option so you get a comment.

Stuff like this can be posted in the appropriate forum. I have no idea if the mods ever read the posts outside the forums.