Through my experience with psychic vampirism, usually you can detect if a psi vamp is 'feeding' from you if you start to feel energy being sapped away from you e.g. feeling fatigued. You may be able to work out who the psi vamp is in the room.. it's from the general direction of where you feel like you're being glared at /watched.

I went through a few days believing i was a psi vamp until i got told a psi vamp is a lifestyle where people consciously and deliberately drain others energy for their own selfish purposes. Silly me =="