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.