A quick question: is there a spell in any of the 5th edition books which will enable you to detect the presence of the Gift? There was such a 4th edition spell, but I can't seem to find a similar one in 5th edition... Thanks!
There is, yes, and off the top of my head it appeared in Hedge Magic Revised. It's just treated as a standard application of the magic detection guidelines.
base 10 +1 touch +1 conc, InVi 20.
Why do we need conc duration? Just make it momentary, that's all you'll need to know if the gift is there...
So that could also be InVi 15 
Presumably so you don't need to cast it repeatedly, likely inside a Dominion aura, and thus risk the inevitable and enormous botch.
You need to recast the spell if you want to investigate more persons so the momentary duration is enough.
That's the case if Touch is the range. If Touch is the target (or hearing etc) then you don't need to recast it, merely maintain concentration.
I presumed that Conc will be needed to look inside enough to detect the gift.
The gift is not as others magics... it's tricky ^^
Would a Hermetic Magus be able to detect the gift simply by supressing Parma Magica? It says that Parma Magica prevents Magi from noticing the effects of the gift (-3 to social skills and all that). Wouldn't it stand to reason that dropping the Parma Magica means they would notice said gift? "Time to send out a ping. Dropping Parma... now. Say, there's something odd about that guy. I should investigate further."
I don't think this effect is something that is instantly noticeable, like a change in their appearance or a nauseated feeling suddenly manifesting. I think the Gift makes every comment a tiny bit more suspicious, every glance a tiny bit more furtive, every pause a tiny bit more damning - so that over the course of one conversation, it adds up to a penalty to social skills. Since magi probably all know about these effects of the Gift, I would allow a roll (Perception + Awareness?) after one meaningful interaction, to see if they notice that their reactions to the person are more negative than they seem like they ought to be. This wouldn't prove the Gift, but at least it would give them a reason to check via magic.
Frankly it seems like suicide to lower your Parma to detect whether someone is Gifted.
"Hey look, that person might be a devious Gifted enemy. I know, I'll lower my Parma and attempt to strike up a witty conversation with them. If they don't laugh at my jokes, they must be Gifted."
Also, as gerg suggests, I don't think you can tell just by momentarily dropping your Parma that someone is Gifted. If you want to notice the social inteaction penalty of a character, you need to interact socially with them, without your Parma on. This should require a significant length of time, at the very least holding a conversation or equivalent.
Even then it would be hard to tell the difference between someone who was odd because they were Gifted, or for some other supernatural reason, or merely because they were in fact an odd person. The other problem is that even if this person isn't Gifted or otherwise weird they might behave oddly interacting with you because you are Gifted --- that -3 to social interaction is a pain.
If you want to try to detect odd people like this, you would be better off sending some grogs to go talk to them and report back. It's less risky and hopefully your grogs aren't too weird themselves.
Salvete,
don't forget about the Magical Air Flaw 
The odd feeling about a person just gives you a hint, but it is in now way a proof for the Gift.
And you woudn't detect The Gently Gift either! What a pity for Jerbiton Magi!
regards
Widewitt
Salvete Sodales!
That Gift-detecting spell in Hedge Magic uses concentration duration because it is not a Intellego-spell aimed at a specific target but a sense-spell with target touch. So the concentrating magus could check some people in a row by touching one after the other.
Vale,
Alexios ex Miscellanea
Hm, honestly I'm not sure if I like the Gift detecting spell. It seems to take the story out of the quest for an apprentice. I liked the formerly held idea of having to keep an eye out for those outsiders who had strange stories connected to them, and tracking them down to determine if they were indeed apprentice material.
With that spell, an item of Apprentice Detection is just a season away.
Does anyone else feel this way?
Except, of course, that without a similar spell, you might get a season into trying to Open the Gift of your apprentice to discover that actually, they're just weird and unlucky. This is, itself, a fun story, but it seems unreasonable not to be able to detect the presence of magic in a person. Besides, you still need to actually cast the spell or use the item on a person - it's not the method but the act of finding the apprentice that counts. Even if you just send a grog out to search, that should be a played-out story, and also gives the other players the fun of choosing the pacifist as the potential apprentice for Homocidalus ex Flambeau. I find the idea of a magus spending time making the item and then commanding an expedition to use it more compelling not. And, after all, you can always sell potential apprentices to other Magi. "For five pawns of Ignem vis - yes, we all heard about your little run of luck there - I can tell you of a village where the smith's son woud make the perfect apprentice for you. Mind you, I might get a better offer from someone else." It's lovely Tribunal fodder.
Prefer using a considerably higher Spell Effect Base Level. That makes it a bit of effort to work and only specialists or skilled magi will be able to cover large areas or many people in short times.
Thanks all for your replies! My copy of Hedge Magic is currently loaned out, so no wonder I didn't find the spell...