How did they know she was a virgin?

My guess is that she yelled "I'm a virgin" or something when his kidnappers implied she would get sexually assaulted.
I don't mean to impose, but I am the Ocean.

Well when she is talking to her friend when they first get to paris apartment her friend says "Well you gotta lose it sometime" So if the guy who gave them a ride there planed a but in her purse then they would have been listening the entire time and heard her friend mention she was a virgin.

gynaecological exam, I'm guessing?! =P

They took the "fresh" looking ones and sold them to the rich guys, right? They advertise these girls like products, say whatever things about them that will increase their value. Maybe things like being experienced, willing, tight, or pure/virgin. Perhaps the kidnappers heard her claim she was a virgin, as she shouted it in attempt for them to gain some sort of empathy that they may let her off the hook... but of course they realized "hey this one may have some value" so they put her up for sale along with the other ones.

There may not have been a physical test, but if there was, I would imagine one of the jons tried to get it in but couldn't because she was too tight... and that's how they found out. Emphasis on the failed part, because they did say "certified pure" and in the next movie, she is still a virgin.

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She does not have to; just how many advertisements you watched on TV is true.

Trust me, in that part of the world, you just need to BS out of your way and just get the money.

Plus once those buyers get whatever they want, they really don't care. They are filthy rich and 'virgins' can be only used once anyway. They bought her out of attractiveness and 'virgin' is just a brand to them.

Traffickers find that out rather quickly after capturing the women. To the buyer a virgin is worth a higher price as most would prefer "new merchandise" since they want control and the chance to have them first. Every woman and young girl is usually examined by a recruited docotor or nurse or sometimes by the women who work with them (they are groomed to know these things to help up the price). This isn't the case everywhere but in most brothels of Europe its a requirment.
Bunch of sickos:p

Also, they may not really have known but just claimed that because that made her more valuable.