How do you try on thongs at a store?

Yeah so I have never bought a thong before and I probably won't. But I was just wondering for all you out there that buy thongs, how do you try them on at a store? Because you would think that it is kind of gross and unsanitary, seeing as how the little string goes up your . Are you SUPPOSED to just try it on like you would regular underwear or a bathing suit? Cause I dont know how else you would know if it fit, but I am just grossed out thinking about it...