Swimwear, yes, but over your own underwear. At least that's how you're SUPPOSED to do it, but I'm sure there are some oddballs who don't care to follow that little bit of etiquette.
I've never tried on underwear and I wasn't even really aware that people do this, haha. But apparently they do. I just know what size I wear, though I can usually wear a size smaller as well, depending on the individual pair. And you can usually tell if underwear will fit just by eyeballing, it if it's your size or a size up or down, depending on the style and material. If it doesn't stretch much, you stick to your size. If it does, then you have a bit more leeway.
Bras, however, should always be tried on. I'd never buy a bra without trying it on first because I'm very picky about how they fit. All cups are NOT created equal. =P Give me ten different bras, all in my size and in styles/designs that I like, and I still may only like one or two of them after trying them all on.
Yes, but its proper etiquette to try them on over your own underwear (so really, it's no different than if you were to try on a pair of tight pants).
The only way to be sure that someone else has not already tried on a pair of underwear before you buy them is if you get them when they're first put out onto the sales floor, or if they come in a sealed package, so make sure you wash your new undies before you wear them.
Yes we try them on. I work in a lingerie store (we also sell bathing suits). Women are asked to wear there underwear when they try on underwear or bathing suit bottoms, we also have posted signs,. Most women do tend to follow this guideline as they probably don't want other womens va jay jay sauce on them either.. BUT you do get the odd few women who don't seem to care and just go about trying the stuff on like they have owned it for 6 years and are just gettin dressed on a sunday morning...
i don't.. but if you want to there's a sign in the dressing room that says to leave your underwear on underneath the bathing suit while you try it on.. I don't know about underwear though.. who trys on underwear before they buy them?