It's different for every girl but that's some good advice above. I've had a lot of luck bringing up 2000s fashion but every girl's different, some will be more open and some won't. You're gonna have to figure out just how willing she is to talk about thongs with a dude in the first place. Sometimes if I wanna test how comfortable they are I'll make a "typo" where I write "thong" instead of "thing" and get her reaction. For example, with one girl we were talking about her most recent instagram post so I said "I saw the thong you posted," make sure you make it believable that it was a typo, act all sorry and that you meant "thing". If she laughs it off and makes a joke about posting in a thong, showing off her thong, etc. then she'll probably be super comfortable and you can bring them up candidly again. If she doesn't pay much attention or has a negative reaction for some reason then you can try to start a thong chat but it might be harder.
Like I said before 2000s fashion is a good gauge of which girls are into it. If you manage to bring up fashion and maybe crack a joke about 2000s fashion and girls showing off their thongs, their reaction will be a pretty clear indicator (if they say that fashion sucked might be harder to strike a conversation, if they like that fashion or don't mind it that's a better sign).
Like I said though every girl is different, some girls may be shy but be willing to talk about thongs or vice versa. Best advice I can give is make it seem really candid and natural, don't bring it up too many times or she might start getting suspicious, and the most important, try and see how willing she is to talk about thongs before bringing it up (the method depends on how close you are to them, if you're not too close I would say use something like said above, like you saw a girl's thong and don't know how they're comfortable or something, it's more extreme but if you're not as close then it won't be as bad if she finds it weird. If you're closer, then a little more subtlety might help so you don't make it weird/mess up your relationship down the road, and if everything lines up you'll be able to bring them up multiple times without it being too weird).