Understanding the mental state of the people around you is called "society". It's not particularly easy, which is why it takes us years to learn it.
But if you need a simple heuristic to not get in trouble at a conference, here it is: do not make sexual advances to people unless you are 100% sure they are welcome. Or, if that's too hard, use this one: do not make sexual advances at conferences. Seriously, it's not hard. You can hop on OK Cupid when you get home.