It is a very difficult question that lingers on peoples minds. Half of them wants to get to know them, date them, and become close to them. The other half of them is in fear of rejection. How do you tell who really likes you and who doesn't.
I've formed a few good tests to see:
Drop something while your next to the person you like and see if they pick it up for you. While this is unreliable, often when a person likes back they are more likely to pick it up. The reason of this is, if the person is not attracted to you then they shouldn't be paying to much attention to you and they wouldn't notice something as subtle as a dropping of a pencil or something else. If they are attracted they they will notice everything to the slightest sound and touch and they will pick it up.
Make them touch your hand. Try to give them something and if they purposely reach closer to you and touch your hand even when there are much easier ways to grab it. They take it and perhaps linger near your hand mabye a second or so longer than any other person would.
Laugh, usually if someone is attracted to you they will also laugh at the same thing even if they don't find it funny. This would be because they would want to show you how they are similar to you even if they aren't.
This also works with smiling, if you smile for no reason a person that is attracted would most likely smile to because they are happy that your happy.
Make lots of eye contact, if they keep eye contact for awhile then they might like you and if they take it away immediately they they might like you. On the other hand if they look at you like, "Hey, sup" instead of, "Hey...." then they probably don't like you. IF they look away immediately they might be nervous and if they stay for awhile they probably are more outgoing and are trying to show you, I LIKE YOU!
These are just a few pieces of advice, thanks for reading and I hope these helped