If you catch them looking at you again, and again, and again, and again, and again it's a good sign (not a guarantee though). And I'm not talking about on the same day, I mean if it goes on for weeks/months, they obviously are attracted to you and want to look at you whenever they have the chance.

If the person continuously goes out of their own way to meet up with you, or arrange to meet up. Even if it means hanging around for long periods of time.

I've had both girls and guys do it to me, and I've done it myself