It seems like girls usually make their disinterest clear. Even if they're trying to be polite and refrain from straight out telling you to leave them alone, they give off a disinterested vibe through their voice tone, body language, and conversational engagement. You'll probably be able to tell without consciously looking for signs. But if you want some specifics...
Eye contact failure:
Making an obvious effort to avoid eye contact. Refusing to acknowledge you. Rolling eyes or looking up or away if she does make eye contact (looking down would be submissive, looking up means she sees herself above you). Looking around during conversation as though looking for some escape.
Refusing to engage in conversation:
Answering questions blandly, in as few words as possible (don't make this easy for them, avoid yes-or-no questions!). Asking no questions in return. Adding nothing to the conversation unless prompted. Speaking in monotone, with no enthusiasm; bored, disinterested, offhand voice tone as though she could care less about the conversation and clearly doesn't want to talk to you.
Negative body language:
Distancing herself from you. Turning away from you. Crossing her arms or legs. Not smiling, or using a weak, forced smile. Recoiling from kino, refusing to reciprocate.
Of course, not all these things gaurantee disinterest. Just because she's crossing her arms or isn't touching you doesn't always mean she's not interested, but the more of these behavoirs she's exhibiting, the more likely it is she's not interested. I'd say the eye contact and conversational signs are the strongest indicators. But the only real way to find out for sure is to "go for it" and try to make a connection (ask for her number or screenname or try to set up a date on the spot). As they say, anything but a straightforward "yes" means no.
For example:
"I don't give out my number to people I don't know."
"How about you give me your number instead?"
"Maybe later"
"I have a boyfriend"
"I'm really busy"
"My phone is broken/lost."
You get the idea. But often even if they're not interested, they'll still give you the number just to avoid directly rejecting you. If this is the case, they will do things like:
Giving you a fake number. Never answering their phone when you call. Never returning your calls if you leave messages (I usually don't even bother to anymore). Answering, but displaying the conversational signs of disinterest. Evading your attempts to set up a date. Failing to show up if you do set up a date, or bailing at the last minute.
By the way, when you do try to set up a date, take care of all the details at once. Have a plan beforehand and make all the decisions yourself; she won't make any herself anyway, or if she does then she won't like having to do it. If she balks at your first choice, have another one ready. If she won't agree to anything you suggest, then she's probably not worth pursuing. This is a mistake I used to make a lot. I might call on a Tuesday and get her to agree to hang out Friday, but rather than saying "Ok, I'll pick you up at 8:00 and we'll (insert activity here)" I would say "Ok, I'll call you on Friday and we'll figure out what we're gonna do." Needless to say, the Friday call would go unanswered every time, any messages unreturned. Without making secure plans, she undoubtedly will have made other plans by Friday.
Don't worry about looking for signs of positive interest. If you're getting them then definitely go for it, but even if you aren't it could just mean that she's hiding them. Just assume that she's interested until she demonstrates otherwise. Keep trying, using confident persistence, until it's clear that she doesn't want to see you. Don't be afraid to move on when she does. Remember to guage her on her behavior rather than what she says.
Also, flirting with other guys in front of you is not necessarily a sign of disinterest. She could just be responding to whoever seems most attractive at the time. Or she could be testing you, waiting to see how you handle the situation. Will you give up and let him have her, or will you prove to be the more dominant and take her right out from under his nose?
