Usually in my country girls always wear underwear, both panties and a bra. That's really normal, no one's asking for the reasons, but I've to admit, that our girls're a bit shy. So it's usually really strange when you see someone without a bra, but it's not impossible!
But how do you recognize a girl without a bra? Well, it's really simple, you'll either notice that she doesn't have straps on her shoulders, or - which's much better to look at - her nipples can be seen purely through her dress! But try not to look at it so conspicuously, otherwise the girl may notice you, and I think that won't be so good to you later.
Although this is just a small-time thing, people, especially men like it, when the subject's a really hot and naughty girl, and doesn't wear a bra. My best memory about this was several years ago, when I was in a house party, with a lot of people. When the next morning we all tried to get home, some with hangover, some still drunk, we sat down at the bus stop, waiting for the bus. There was a cute, young girl, a teenager, who was wearing a white top, but no bra under it, and when she bent over a bit, I could clearly see everything of her tits! I could easily notice their shape, their size, and I also saw that she had quite nice nipples! I didn't tell though the girl, that I saw her tits, why would I do that?
Sometimes girls choose not to wear bra simply because they don't like them, that's normal, too, I mean I can understand them. It's a really funny and naughty thing as well, so guys, can an eye out for those nipples, and you may have a happy day!