it's easy, just ask something like how much experience she has because you only like girls with experience. she'll feel encouraged to tell the truth about how big of a whore she is. if she says she's a virgin, try and discourage her by talking about how being a virgin is weird. if she still persists, it means she's either good at lying or she cares about morals and she's actually a virgin. even if you don't ask her this way, she'll give off red flags if she's not a virgin- e.g. tattoos, impulsive behavior, sexualized, has male friends on social media, etc.
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