To detect if someone is looking at you without directly looking at him/her, give out a slow, methodical and convincing yawn when you think that person is looking. Yawns tend to be visually contagious, causing someone people to also yawn as a reaction – it can be a subtle action or a fully expressed yawn.
Catching someone looking at you without them noticing you noticing them is useful for anti-surveillance, counter-surveillance or for everyday romantic encounters.