You can tell my body language. If you are on line and never met, then a meeting should be arranged and you can usually tell within the first 10 minutes or so who's into whom.

When I clicked on this, I did not know you meant online. That is a more tricky question... I guess I go by how warm/genuine the replies are, and how readily the person replies. When I am drawn to someone, I tend to be thinking about them, possibly checking on line frequently for replies if we have a back and forth conversation in play. Likewise, I will tend to reply quickly (circumstances allowing. I DO have a life in the real world: work, friends, family). My interactions might also lean a bit more toward flirty, maybe even subtly suggestive when I am intrigued romantically.

They return your attention in kind. If you message them they message you back, If you ask them out and they say no they offer an alternative.

Seems to me the direct approach is always the best, just ask them. Life is, too short to pussy foot around.

lmao, its usually very obvious once they are

For me, it's when they actively communicate. If they don't message or respond I usually take that as a no go. I'm big on not being the only one to start a conversation.

I wish people would be more honest, but I certainly understand the fear of rejection. I just told a guy on FB that he was intelligent and cute. He hit like and if it is not returned, that is OK. I put it out there and we can just go on as FB friends. But in "real" life it is more difficult. I wish it was easier.