As a woman who goes outside almost every day, I'm used to people looking at me for all kinds of different reasons. In certain cases, it can be harder to tell who's looking where: on a crowded train, or across a packed bar. If you're unsure if a person is noticing you, there's a little trick that I have found to be very effective:It's called mirroring.
In a 1999 study, psychologists defined mirroring as "nonconscious mimicry (无意识模仿)of the postures, facial expressions and other behaviors of one's interaction partners". It's long been used as a strategy that successful people can employ strategically to show their engagement (参与度) during job interviews or at work. They believe when done on purpose, mirroring can help people understand each other's ideas or feelings very well.
It's a proven method. In an experiment, volunteers were asked for their opinions about a series of advertisements. A member of a research team mirrored half the participants, taking care not to be too obvious. A few minutes later, the researcher "accidentally" dropped six pens on the floor. Participants who had been mimicked were two to three times more likely to pick up the pens. The study concluded that mimicry had not only increased goodwill towards the researcher, but also contributed to an increased social orientation (取向) in general.
If you want to know how to figure out if a person is looking at you, just throw out some kinds of gestures and see who copies you. But not everyone uses mirroring to figure out who's making eyes at who. My years of experience of analyzing the messages sent consciously and unconsciously by the human body has left me with particularly good powers of observation. However, the average person has not had the same chance to improve this skill.
Of course, once you've figured out who's watching you, it's up to you and the watcher to decide what happens next. Not everybody watches with intent or enjoys getting hit on in public, so knowing how to react properly is very necessary.