> pay attention when you hear someone’s weight to see if it matches your heuristic

I don't think I've ever heard an adult male ever mention their own weight in public. You can see why it'd be hard to estimate when you don't have any data.

You’ll have to get a job at a county fair running the weight guessing attraction to get enough data. There’s no other way to do it, it’s how we all figured out how to estimate the weight of a human. I’ll call my friend Chucky, he’ll get you a gig in a traveling carnival for the summer.

Or simply any job that requires checking IDs. Bouncer, bartender, cashier in a store that sells alcohol/cigarettes, etc.

Huh, I have never seen weight specified on any ID.

Every US ID and driver's license I've ever seen (and I've seen many) list height and weight.