You'll know.
I have this friend, he's 60, and he described meeting her and the incredible connection they had. They talked for hours on a bus back from some activist thing.
The next day she approached him asking what was that? What happened on that bus?
He lied, because he was married to someone else. He said 'nothing. We just had a conversation', and he never saw her again.
This is something about which it's neither possible nor desirable to rationalise; its like a religious faith - romantic love is the real faith of our time. It's our God. We need it as human beings to function.