How can a girl tell if a guy likes her online?

Online and sure are two words that don't go well together unless you are very both very good at writing. Even then, the tone and the expression are missing (smileys don't cut it.)

First off,
how trustworthy is this guy?
i believe it is possible to tell, but it is tricky.
if you know this guy, and see him fairly often, it would make it a lot simpler.
generally, does he start conversations? does he send the first message?
does he ever compliment you online? are they genuine?
its pretty much the same as in person, but you have to be careful. as ashley-205 said, the guy could be just pulling you leg.
i wouldn't take any online thing too seriously, unless you actually know the person. and if it seems like this guy does like you, you should ask them in person about it. not online.

I don't think there are to many ways to tell if a guy likes a girl online, you never know when there just pulling you leg or if there telling the truth.