It’s not super difficult if you have some sort of temperature gun. Take measurements on the hoses and block to see if coolant is circulating.

Thermostats can fail in 2 ways.

They can fail open, which causes you to never get up to temperature. This isn’t good, but it’s not gonna risk catastrophic engine damage. Although your engine not getting up to temperature can accelerate premature wear, so it should still be fixed.

They can also fail closed. This is the one most people think of. Coolant simply can’t circulate properly and your engine overheats rather quickly. This is what causes bad head gaskets and the like if not spotted.

I’ve seen both types of failures multiple times, and had it happen to me once (failed closed in my case). It’s not implausible that he’s only seen failed opens, it’s just a bit strange.