Well, depending on the person, they may do some of the following.
-stare at you until you catch them, then continue staring when they think you've stopped looking at them.
-constantly bump into you or do something to give them an excuse to touch or be close to you. They may complement on something like a necklace, then go to feel it.
-ask you for things they may or may not need constantly, and not ask any one else.
-tease you.
-they may be a little territorial over you, or overprotective.
-they seem to talk/joke/mess with you more than anyone else.
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If they flirted with you, or even if they liked talking with you about anything. They could just be checking you out over and over, and if you ever caught them doing that, that should be a big clue. They don't have to check you out and then speak, because sometimes not speaking means that you make them nervous, which is a definite green signal for go, if I ever heard of one. If a guy looks at you all the time, talks about you (good things) to other people, likes talking to you, tries to show off in front of you, and comes close to you, that's a total clue that he likes you.
1) well if they are in a deep conversation with someone when you enter the room, they come straight over to you and start taking and ignore everyone else. 2) they flirt with you. 3) they try to make eye contact often. 4) give you the eyes. 5) trying to get your attention.