How do you ask them? Why do you even care how many guys she's slept with? It's your turn, that's the only thing that should matter.

And most importantly, why would you even believe her? IMO she's most likely lying. I remember asking a girl once who was 18 and she said she was a virgin. Then later she told me she was lying.

If it's really just your turn, and she's probably lying, why even give a shit? And if you are screening for an LTR, how can you really know that she's telling the truth?