How do you know if someone is wearing a thong?

Someone else said "You don't; it's none of your business what underwear a person is wearing."

You can usually tell by looking at their butt. and depending on how tight their clothing is you can also see panty-lines that will tell you anything. But if you've ever seen anyone who has gone commando (underwear-less) and seen how their butt was carried then you can also say that someone is wearing a thong because it is very similar to that. because the thong lacks coverage of the backside so it doesn't give any support to it either.

At least that's my theory. obviously if you are gonna go people watching and staring at butts be discrete because some people can be seriously offended by it. invest in some dark and possibly reflective sunglasses. big ones too.