How do you know if you have fallen for your friend with benefits and how do you tell her you may have feelings for her?

"Friends with benefits" in my opinion is an act of covering up true feelings. Anyway, just tell her how you really feel and let her know you want to keep doing what you're doing, but as an "item". It'll probably feel really weird because nothing will change since a relationship has started and the element of surprise of "moving forward" is gone.