"Comment vas-tu, mon amour?"
* "Comment vas-tu?" means "How are you?"
* "Mon amour" means "my love" and is a common and affectionate way to address a girlfriend.
You could also say:
* "Comment ça va, ma chérie?" (How are you, my darling?)
* "Comment vas-tu, ma belle?" (How are you, my beautiful one?)
These are all very common and loving ways to address your girlfriend in French.