It depends on how it comes up.
If you are joking around about something and he or she says something like, "I just love you," you are OK saying nothing.
If you andyour partner are having a serious conversation about your relationship and he/she says something like, "Look, you need to know something. I love you." Then, you need to be honest and totally upfront, even if it ruins the moment. It is best.
If you don't know how you feel, you are not in love with them. Period. And, it is best to tell them that. It will hurt their feelings, but it will save you a truckload of heartache later on. I promise.
Good luck. I know how you feel.