With a successful Perception + Awareness roll, difficulty 6 , a character can feel magick in use within her immediate vicinity. Really powerful Effects might lower the difficulty as low as 4, while unusually subtle ones could raise it as high as 10.
The Prime 1 Effect Sense Quintessence can detect "residue" of magickal acts after they occur. Small feats "dissapear" within minutes, but really strong ones can be felt for weeks afterward. The more time has elapsed since the casting, the higher the difficulty of a Perception + Awareness roll will be.
(ref MtA pg 172)
1 succ = Someone nearby is using magick
2 succ = Someone nearby is using 'insert primary sphere here' magick
3 succ = This person nearby is using 'insert primary sphere here' magick
4 succ or more = This person nearby is performing 'specific effect'