I recently bought a TV and discovered that MTV is nothing but shows about kids having kids! What ever happened to the music? Matt Pinfield? Kurt Loder?
Your joke is a good variation on "how can you tell if someone is vegan?"
Hadn't thought about vegans, but you're right, they're prone to the same joke. However, I do know vegans who don't make a point of saying it, whereas all the no-tv people (ALL of them) had the urge to mention it, although this was more in the 90's, when TV was still what the internet is today, entertainment-wise.
Here's one of the few jokes I ever came up with:
Q: What do you do if you want to find out whether somebody owns a tv? A: Nothing. If they don't have one, they'll tell you.