im an expert at it lol. I can tell if a guy or woman is wearing a thong, panties, briefs or no underwear. Everything from how the fabric of the pants rides up ur crack as u walk to the see through yoga legging pants etc.

oddly enough i have noticed a few dudes rocking thongs before. and all of them were wearing women thongs too not mens. one dude i noticed in the summer was wearing soccer shorts and the shorts were yank up his butt crack just like a thong would so that was an obvious sign then i was able to tell bc the top of the thong (on the back) was visible since the shorts were very thing fabric. and then to top it off a bit of the waist strap of the thong exposed itself above his shorts and it was a pink color.

some other dudes usually are wearing sweatpants or tight jogger pants when wearing thongs from what i noticed. this is even easier to tell cause the jogger pants are tight around their butt so any fabric material under neath will be visible (the outline of it) and u can see the outline shape of the thong, plus like i said before the sweatpants is normally yanked up theier butt which is cause by wearing a thong