how can you tell (more towards females)

Women are only complicated because we dont think like them. They stand looking at you across the room and think thats enough of a sign to the man that she likes him. However men dont think like that so it doesnt work. Basically they do thinks they think works at attracting men and often its totally lost on us.

best way to tell is if she laughs at your jokes, seems to always find a reason to touch you and makes alot of eye contact.

As for standing across a room and knowing... couldnt tell ya. As for meeting someone for the first time and having a conversation that was amazing and knowing something would work out. Yea thats happened with me.