Girl ask his boyfriend how he knows if a girl had sex with other guy.. my question is what's trying up to ?

How old are you both and what's the nature of this relationship? That's important to know in trying to figure out what the heck she is up to, and how you perceive her. Between just very young friends such a question could be just curiosity, as girls often ask their guy friends weird questions about other boys.

Most young guys couldn't tell a virgin from a whore, and probably wouldn't trust either of them. What's your story?

There could be several different reasons he is asking:
1). Guy B has a STI, and GUY A wants to know if she has it.
2). Guy A wants to know if Girl is a virgin.
3). Guy A wants to date Girl, but wants to know a little about her past first.
4). Guy A wants to ruin Girl's reputation.

The best way to find out why he is asking that is to ask him why he is asking.

In the end the number of partners a person has means precious little.

Also, Please answer Tal's questions.