One way to tell if a flirt really likes you is if he doesn't flirt with you. If he's a natural flirt, he's probably not putting too much thought into his actions when he's friendly with other people--that's just how he is. But if he clams up when you are near, acts fidgety and nervous and not really like himself, it could be because he is now very aware of his actions and is overthinking everything.  Watch him interacting with a group of people, then walk up and say hi. See if his behavior changes--if he gets quiet, tones down his goofiness, or doesn't say much but keeps glancing at you. See if one of your friends will join the group and try to embarrass him a little--nothing mean, just a small joke at his expense. If he blushes or gets embarrassed when he'd normally take the joke in stride, it might mean he doesn't want to look bad in front of you. If you make him nervous, he may continue to be very friendly with other girls and appear to ignore you or be less friendly toward you. Not every flirty person becomes nervous when around their crush, so if they're not suddenly getting shy, it doesn't necessarily mean they don't like you. They might be the type that will give you more attention instead. If she's interested in you, she will subconsciously copy your body language. When you're hanging out, try crossing your legs and see if she does the same a few seconds after. Take a drink of water and watch for her to also take a sip.   Mirroring a person's actions is a way to bond, get comfortable with each other, and send the signal that you like the other person, even if you're not even aware you're doing it.  If you decide to try this in reverse - mimicking her movements so she will subconsciously feel comfortable with you--make sure you don't follow too closely or precisely. Wait a few seconds before mirroring her actions. If she picks up on what you are doing, she may think you're mocking her and the effect will be ruined. Try to notice if she's constantly glancing at you across the room or when you're in a group, then blushing or glancing away when you catch her looking. When you're having a conversation, notice if she starts looking into your eyes as though she's studying them. Prolonged eye contact or a lot of quick glances in your direction indicates she's interested in you.  To see if she likes you, try holding her gaze while you're flirting. If she seems uncomfortable and looks away quickly, she's probably not interested in anything romantic. But if she stares back at you, it's a good sign that she's into you.  You can find out if she's watching you by taking a small action, like appearing to look at something strange out the window. If she's watching you, she won't be able to help but look out the window, too. When you're  with a lot of other people, does he seem to pick you out of the crowd? When you're having separate conversations with other people, does he suddenly chime in, alerting you that he's been paying more attention to what you're saying than the person he's talking to? If he likes you, he will constantly be looking for ways to be near you and interact with you.  Go out to dinner in a group and see if he makes sure he's sitting next to you. Try to hang out a few times in group setting, and notice if he makes a point to come talk to you each time. Go to a party and try to stay until the end. If he hangs around with you long after most people have gone, he probably went to the party just to talk to you.
Notice if he becomes nervous or suddenly quiet around you. Watch her movements when you're together and see if they mirror your own. Look for lots of eye contact. Get into a group situation with him and see if he pays more attention to you.