This one REALLY throws people off, they rarely have a quip or retort to answer. “Too real” is the most common in my experience
Either "I'm not dead yet" or "I'm still alive" with a somewhat halfhearted "I guess..." Trailing behind.
Once upon a time I was a little more cheerful and gave answers like "Celebrating my streak of consecutive days lived".
I used to say "alive & well, and living in America!", but about 10 years ago people started saying "I'm sorry" instead of how they used to respond with something like "Fuck yeah!", so I stopped saying that.