Going outside of the official rules, I played in a live Ars Magica game many years ago that had a couple of demon hunters amongst the magi. This idea may have been original or taken from something written that I don't own, but they did their thing by creating a white flame that burned brighter as you got closer to a demon. The downside was that you could only cast this by calling upon infernal magics in the first place (essentially, the spell drew its power from the very presence of the demon; no demon nearby = no flame) and the possibility of corruption from using such naughty magics was very great. I always rather liked the idea that to take on the demons, you had to corrupt yourself first. Unless you were properly holy, of course 
If you're a holy character, Invocation+Adjuration will reveal demons easily.
yeah I was thinking of that but taking two major virtues just for a single aspect of a character is a bit overkill, also rather frustrating.
Regarding the Sense holiness & UH skill... Should be Perception the only stat used... detecting a demon?
It could be posible, for example, using Communication / Intelligence... perhaps even some physical stat... to detect a demon with this skill? Lets say you are talking for a long time with a demon, perhaps a COM+S.H&UH could do the trick? ... INT trying to compare your general knowledge on demons on the strange things this guy does?
Spamming the room with an Infernal-aspected PeVi effect will, as others have noted, identify demonic illusions...simply by having them disappear. It does require that you have a pretty high PeVi total though - just to get the level of the spell high enough to have it even moderately compex illusions. You could also use the specific version, if you were fine with it affecting "demonic illusions" only (as opposed to everything Infernal).
This is, of course, also similar to the PeVi way to identify a demon: shoot everything with Demon's Eternal Oblivion, and observe what (if anything) starts smoking and screaming - of course, that'll get you both demons and faeries pretending to be demons, but them's the breaks.
The main issue being of course the fact that you'll run into individuals that don't care for you casting random spells on them - like other magi.
Likely a variation of "Winds of Mundane Silence" - it blindly affects everything in a room, rather than specifically targeting something in the room. You'd have to have a AoE effect to make it more useful than for just inside a room, of course. (Like having Bjorner sensory magic - "everything that can hear this effect is affected" sort of thing.
Take a level of True Faith - it acts like the Gift for Holy virtues, and as such allows you to learn 10 level's worth of virtues in a given Holy tradition, for free. (And by "free", I mean "get your GM to set up a story arc for it.")
Any magus can make up new non-standard RDTs without new virtues or mysteries. Have a Special Target based on Room (so worth +3) to affect a "room's worth" of stuff without a room for use outdoors.
Or something.
Rich.
But only within the limits that the troupe sets as appropriate. And it should be cool and medieval/cabbalistic sounding, of course.
Something like "a room of area but outside" is a bit boring.
How about Perception+Infernal Lore?
First day a player created a companion with Perception -3 and sense holiness +5... was pretty funny. I ve offered him to change his perception score, as we are just starting and learning how the game works...
So i was wondering if some other stat could be a combination (Int/Com...) useful for that virtue... Although the virtue CLEARLY stats for PER + SH.