Most Helpful Girl
For me, I know I'm in love when I want to throw up thinking about how I'm going to see them and then, paradoxically, once I'm with them, feeling more at ease than I could even imagine feeling. Although that might not be helpful to a case where you aren't seeing each other in person.
Just be yourself, you'll know if you really love her when and if you do. Right now you can just answer kindly. The context is important to understand what she even means by it. If she signs off with it, like "bye, love ya," or slips it casually into conversation it's most likely platonic or just testing the waters to see if the potential for romance is there.
An I love you confession of a romantic nature (where she is sure she has serious feelings for you) is usually the center of the discussion. And it usually comes with all the pronouns and correct spellings, like "I love you" vs "love ya."1
Most Helpful Guy
There is a major difference between infatuation and love. Infatuation feels like love, but its far from it.
You know you truly love someone when you're willing to sacrifice whatever it is for their own benefit and happiness even at your inconvience and with joy.1