You'll never know the answer. And even if she's an actual virgin, you'll never know how many other men have been in her mouth, other holes, etc.
However, you should be experienced and attentive enough to notice red flags. Especially in the beginning stages where you're just having fun and she's really into you. If she's comfortable around you, women will let you know what you need to know. What does she dress like? What's her friend circle like? Are they all hoes who party every weekend? What does she like? What's her education level? Perhaps most importantly, what's her family situation like? (Relationship with father).
Get the answers to all of those and you won't have to worry about bodycount.