Below are commonly asked questions about this spell. Some of the information is pulled official D&D sources (such as books and the twitter feeds of D&D officials), but other information is derived from forums and online discussions. As always, it is up to the DM to decide how they wish to spell questions. If you have a FAQ about this spell that you feel other DM's may wish to know, please send us the message!
Would a creature with Truesight know that the creature they are seeing is Invisible?
Truesight specifies (PHB, 284).
A creature with truesight can... automatically detect visual illusions and succeed on saving throws against them, and perceives the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the creature can see into the Ethereal plane.
Invisibility is an Illusion spell (PHB, 254), and Truesight automatically detects visual illusions and gives you perception of a creature transformed by magic. Therefore, you not only see them, but you've detected the illusion of invisibility or seen the original form of a creature that uses magic of any sort to make them invisible.
Does Truesight or See Invisibility negate the disadvantage/advantage granted by invisibility?
Officially, no. In a Sage Advice Live panel, D&D's lead rules designer, Jeremy Crawford said that even being able to see an invisible creature with magic doesn't negate the second bullet point [of the invisible condition]. You need a spell like Faerie Fire which specifically says it negates the benefit of being invisible. So, officially, even though an invisible creature can be "seen," attack rolls against the creature still have disadvantage, and the creature's attack rolls still have advantage. Note: There are many DM's that argue against this ruling. In the end, it is up to the DM.
Can Detect Magic be used to see invisible creatures?Detect Magic
you can use your action to see a faint aura around any visible creature or object in the area that bears magic...
So, if the creature or object is not visible, no aura would be seen. In addition, the spell states,
For the duration, you sense the presence of magic within 30 feet of you.
So, if a creature (or object it is possibly carrying) is magically
invisible, the caster would only sense magic
within 30 feet. The caster would not know if the magic was resonating from a creature or an object, nor would the caster know the exact location (note: some creatures, like an invisible stalker, are naturally invisible, and would not be "sensed" using Detect Magic