This is for guys only!!!

It's different for every guy. Every guy has his own game and there's so many ways to tell a girl you like her without telling her. Flirting with her, if they don't talk much around you because they don't want to say nothing stupid and look dumb in front of you. Our body language how we act around a girl.

I agree with the previous two comments, when a guy likes a girl, if he is a reasonably cool guy he will tend to do a lot of playful "teasing". At this point it's more about body language and less about what they say, because just in my personal experience, I tend to flirt and make fun of all my female friends regardless of whether I'm attracted to them or not, but I add more deliberate eye-contact and use a little more physical contact (touching the arms like haywire said for example) when I want to let a girl know that I like her.

In addition to what Tatsu1 said: Being inviting but cold, a mix of things, whip up feeling by being more intense than usual, extended eye contact, touching is a good sign - if a guy is doing something like picking fluff off of your clothes every so often or putting their hand on your arm when you're laughing or whatever.

Act like a douche.

The guy asks the friends of this girl he likes if she's seeing anyone or if she has mentioned anything about him. If you sense an indirect approach from this guy who you think likes you then odds are he really likes you.

Let her be your friend ask her and a bunch of people to go some were and sit next to her