I'm surprised you were able to quote that conversation word from word?
Anyway.....mm.... no. You cannot simply look at a person and do a cliche "I got you figured out" thing. How you dress has nothing to do with it, neither does your body language or anything.
Yes...you CAN TAKE A GOOD GUESS BASED OFF THESE THINGS. And sometimes people, experienced or inexperienced, can guess right because certain behavior/looks will fall in stereotypes.
But like all stereotypes, it's not 100% accurate. The Brad guy was just taking an educated guess but from your oddly specifically quoted conversation, it's possible he guessed.more from an overblown ego than from thorough observation.
How I usually guess whether a person is a virgin or not is usually by conversation. If you're experienced, you can easily weed out a virgin based on what they brag about but half of what they say is mostly myths on how sex SHOULD BE.
For example: If you're an experienced female, and you're talking to a group of girls. You know first time experience is different for many girls. But one thing will always remain common: your first time is not your WILDEST time. Anyway, one girl rambles about her first time being SUPER WILD AND AMAZING AND EROTIC.....you automatically get this feeling that tells you it's all bullshit. Its like a sixth sense thing. HOWEVER, as I.said.....it's never 100% accurate. So you could be wrong.
The point is, it's possible people will take educated guesses based off observation and what they know or don't know...stereotypes being a factor as well especially with "virgins". But it's never accurate...and some people will simply bullshit guesses to seem knowledgable when they really aren't. People will generalize....even virgins and nonvirgins. I'm ashamed to say there are no limits.
Now I can't say much on a guy's perspective....but I'm guessing it's one in the same. They can throw educated guesses, but sometimes they can be wrong. From a guy friend I know, he thinks any guy who doesn't walk with confidence or can sex talk in public is a virgin. But that isn't true to all guys.....as the saying goes: "Shy in the streets, but a freak in the sheets"