Uhhmm Im not sure about specific things. Leaning in, focusing on you a lot, catching her staring. If she's really interested in you and asking a lot of questions, laughing at your jokes even if they arent funny....
I think body language is the key thing. Women arent THAT different from men in a lot of ways, just pay attention to whether they seem engaged with you or not.

Online...I would just put yourself out there and tell her you really like her, that you think she's great in X way, and see how she replies. Worst case scenario, she doesnt like you back.