How to tell if a girl is interested on a dating site.?

I've been talking with this girl back and forth for over a week now on a dating site. She's seems really nice, and we seem to be getting along. The other day I messaged her and casually said if she wanted to exchange emails I'd give her mine and we could chat there, but either way it's not a big deal. She said that she'd rather keep it to this site for now, and I didn't push the issue. My question is how can I tell if she's even interested? I don't want to deal with one of those people who says " I'm not here to meet anyone, just to look around." I also don't want to talk to someone who has no intention of eventually meeting, that would be a giant waste of time. Additionally how long do girls usually wait before they give someone a phone number or email address? All I know is that as a guy if I meet someone I'm interested in I'd give them my email or phone number and get to know them better, and from there decide if I want to meet up or not. Are girls just MUCH more cautious, even with email addresses? I'm kind of looking for a female perspective here. (Please don't respond saying that she's a man)