I often wear light coloured trousers, trousers with thin fabric, or otherwise sweat pants and I can say that most of the time, there will be a visible thong line (unless you're wearing a g-string). I even have a pair of so called "invisible thongs" that do not have lining, but the colour still goes through. So if I see a guy without underwear lines, I'd assume he is wearing boxers/boxer briefs.
Either way, for myself, I don't really care if people can see my thong.