I'm actually not 100% sure now that I think about it, but I think what I do is figure out from the intonation/context whether it's intended as a generic greetin

I'm actually not 100% sure now that I think about it, but I think what I do is figure out from the intonation/context whether it's intended as a generic greeting (in which case I'd repeat it back) or as an actual question (in which case I might respond with "oh, not much, what about you?" or similar).