How do you know when you are in love?

I suggest one of the ways is by monitoring when you feel happy or sad. When you are with someone (Skippy), do you feel happy? When Skippy is not around, do you feel sad? If so, this is one of the marks of love. Further reflection will then be in order: what is it about being with Skippy makes you happy? Maybe Skippy likes you and you like being liked. This would not be enough, I suggest, to indicate whether you actually love Skippy her or himself. When you get to the point of realizing that you are delighting in the sheer goodness and well being of Skippy and that when you are sad, you miss the presence of Skippy, then I think you have quite a bit of evidence that: you are in love.