Your biggest tool to know if you’re really in love with her or not. Right after you’re done, wonder:
Do I want to watch a movie with her? Do I want to text her randomly just to find out how her day has been going? Do I want to learn more about her? Am I interested to know what her passion is? Does it matter ever so slightly if we have anything in common? Do I want to meet her parents eventually? Do I want her to meet my parents eventually? Can I go for long walks to nowhere with her? Do I want to travel to different parts of the world and experience life together with her? Do I want to learn her language? Do I care if she’s seeing other guys? Does it hurt if she actually doesn’t have any interest in me? Do I wish that she’d listen to me always? Can I provide a listening ear and a sturdy shoulder for her? Do I hope that she’d accept me, flaws and all? Do I want her to meet my friends and see if they can get along? Do I wish for those perfect scenarios where she can literally see me at my coolest, my best so I can impress her? Will she laugh at my stupid jokes? Can I just please, wonder about a future together?
If you’re mostly answering no, and you lack wonder, then you aren’t in love. You’re not even tempted on a deep level. If it was sex, you’d want her to leave ASAP. It’s just your dick doing the thinking for you.