I usually just kiss her on the forehead or something, or make a joke like "I know, I'm very lovable".
I tend to suggest to people not to make grand confessions of their feelings too early in a relationship and also during break-up. In both of those cases, the odds are weighted against you that the person feels the same way, and confessing feelings like this when the person isn't feeling it usually has either no effect or sometimes a negative effect.
I know this sounds weird but I usually just smile and hug them or kiss them and don't actually say anything back. My boyfriend also dropped that line before I could 100% say it back and mean it and I recall doing that and I think eventually I told him I'm not quite ready yet but he should know that I'm well on my way and cherish him a lot. That seemed to make him content.
It depends when and how they say it. If I knew I didn't feel the same way yet, I'd just make them question themselves,,"Oh you love me already, I must be pretty special". That way they get the indication that I'm surprised that they're already talking about love and I'm probably not going to reciprocate it just yet. It reminds me back when I was like 7 years old a girl told me she loved me and I just said "no you don't" and ran off and she started crying, wish I could have played that one off better, lol.