Telepathy.
If you flirt with them
By seeing how that person acts around that personAnd sometimes you don't because they do not show it at all
You know when you care for someone when you start to get butterflies in your stomach when ever your around them, when you start to have feelings for them, when you go out of your way to help them,
You probably know someone with their same personality and the two are alike, so it's like you already know the second person.