Straightforward issue here: in the D&D 5e version of true polymorph, the creature or object becomes permanently polymorphed into the creature or object of the caster’s choice after an hour has passed. Down the line, if a caster used the detect magic spell and examined the newly transformed creature or object, would they be able to detect magic?
In other words, if a wizard transformed an adventurer into a spoon with true polymorph, waited 61 minutes and tossed the spoon into a drawer full of perfectly normal spoons, would one of the party members then be able to use detect magic to determine which of these spoons is their party member?
Best Answer
Yes, the magic is still active
True polymorph(PHB, 283) was altered and the spell text no longer calls it "permanent". Instead, the spell description now says(emphasis mine):
Since the spell is currently active and dispellable, it is also active magic that can be detected by detect magic. Active magic is active magic, there aren't degrees of it. There is either magic there, or there isn't. If there is, it's detectable. This is not an instantaneous effect whose magic has ended (and thus not dispellable.) It is an ongoing effect, and ongoing effects are active magic.