Depends if it's sincere or not. If it's not, I will ignore it. Just treat it like they're saying hi. You don't have to answer, to fall into their frame.
It does irk me that people have replaced how you doing with are you ok? It's patronising.
I like simple yeah. are you? and give them a really concerned look. Haha.
Fucking hell that's funny!Next time, cut through the chase and ask her if she want's to fuck, then, either you're getting laid, or you'll never hear from her again.