If it someone I don't know I will simply look at them. My look is a masterpiece of "go away" vibes. My sister, on the other hand, will come out of the encounter with a new Best Friend 4 Evah.
If someone says"How's it goin?" I'll say "It's been and gone." If someone says, "What's goin on?" I'll say, "Nothing, honest." And if someone asks, "Top or bottom?" I'll answer, "Peripheral."
I'll tell them I'm fine (because if I'm not fine, I wouldn't necessarily want to detail the reasons why I'm not to a total stranger) and ask how they are.
With the classic head nod?
the "sup?"
or "Nothing."
or not say anything at all?