so to start off, i don't own any bike shorts. I pretty much ride everywhere either in street clothes or regular gym clothes just as transportation.

the question is

how are you supposed to try on shorts in a store if the right way to wear them is without underwear?

For the purpose of trying out, wearing undies in the store is OK.

+1 Also, if you know your size in a particular brand of shorts you can get away with not trying them on.

I just wear regular shorts and a T-shirt.

Yeah, me, too.

Karl

I'm trying to think whether I've seen a bike shop with a fitting room...

Actually I don't think I've ever tried on shorts (or jerseys) before I bought them. I must have just guessed right the first time and now I know my size. Most online retailers have a sizing chart that maps shorts and jersey sizes to your chest and waist measurements and I have found them to be pretty accurate. And important to check because the sizes on bike clothes (i.e. - S-M-L) in no way corrolate to sizes of regular clothes. I wear a large T-shirt and medium shorts and they fit me comfortably loose but I wear a 2XL jersey and L bike shorts and they fit like Superman's costume. I don't wear the cape though, too much wind drag

Same here, unless I'm going on longish (40-50 miles) rides, and even then, I just have a couple of old pairs of biking shorts from my tri days. For commuting/utility riding, I don't need the bother and while I'm pretty much each to his own, it seems like some of the people I see on my commute are a little bought into the whole "buycycling" concept, which I admittedly wasn't immune to in years past.

But they probably think I'm the King of the Freds (or a DUI convict) in my jeans/shorts, boat shoes, t shirt tootling along with friction shifters, etc.

Do you have the requisite old Army field jacket and the handlebars flipped upside down?