What are some other (read details) shortcuts that might help you figure out if the one you're with is 'the one'?

I remember finding out that I’m pregnant when I was with two people and realizing against my will, I guess, that if one of the two was the father, I would have tremendous anxiety. It was one of the fastest ways to know, on an instinctual level, whether I really loved them. What are some other shortcut ways to figure it out other than the conventional ones?