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The detect magic spell description states:
For the duration, you sense the presence of magic within 30 feet of you. If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any.
In this question it is said that detect magic can reveal the presence of a sorcerer. What about other classes, like clerics (whose magic has a divine origin), or wizards?
The Sage Advice Compendium addresses a related question:
Is the breath weapon of a dragon magical?
If you cast antimagic field, don armor of invulnerability, or use another feature of the game that protects against magical or non-magical effects, you might ask yourself, “Will this protect me against a dragon’s breath?” The breath weapon of a typical dragon isn’t considered magical, so antimagic field won’t help you but armor of invulnerability will.
You might be thinking, “Dragons seem pretty magical to me.” And yes, they are extraordinary! Their description even says they’re magical. But our game makes a distinction between two types of magic:
- the background magic that is part of the D&D multiverse’s physics and the physiology of many D&D creatures
- the concentrated magical energy that is contained in a magic item or channeled to create a spell or other focused magical effect
In D&D, the first type of magic is part of nature. It is no more dispellable than the wind. A monster like a dragon exists because of that magic-enhanced nature. The second type of magic is what the rules are concerned about. When a rule refers to something being magical, it’s referring to that second type. Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:
- Is it a magic item?
- Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
- Is it a spell attack?
- Is it fueled by the use of spell slots?
- Does its description say it’s magical?
If your answer to any of those questions is yes, the feature is magical.
Let’s look at a white dragon’s Cold Breath and ask ourselves those questions. First, Cold Breath isn’t a magic item. Second, its description mentions no spell. Third, it’s not a spell attack. Fourth, the word “magical” appears nowhere in its description. Our conclusion: Cold Breath is not considered a magical game effect, even though we know that dragons are amazing, supernatural beings.
Detect magic, like other game mechanics, operates by this same logic with regard to what is considered magical. The spellcasting abilities of creatures (innate or otherwise) are considered "the background magic that is part of [...] the physiology of many D&D creatures". Detect magic is designed to detect magical effects, not the background magic that suffuses creatures or the universe.
Chris Perkins confirms this interpretation here:
Can detect magic detect magic potential of spellcasters even if they're not actively casting a spell?
It's not a wizard detector, if that's what you mean.
For sorcerers, yes.
This is because magic runs through their veins. According to the book's explanation of power source, they are magical beings. See PHB 99.
Wizards themselves, no.
Unless they are currently affected by or concentrating on a spell, a wizard is just a mortal who has studied how to pluck and weave the threads of magic. They themselves are not a magical creature. See PHB 112 and others for a wizards' interaction with the fabric of magic.
What about all the other classes?
Those who do not use magic are an obvious no but others are less clear. To be a warlock, one must have a magical bond between themselves and their patron, but so far the 5E books appear silent on whether or not detect magic would detect it.
Disclaimer
These answers apply to 5E and the book. It hardly needs to be mentioned that a DM is free to deviate from any book as far as their players are willing to accept.
