Don't sweat such things. As long as she's faithful to you and following whatever other rules you set for her it doesn't matter. Dont get caught up trying to read her mind or being paranoid that she doesn't really like you.
Chances are if she's just a slut she'll be done with you quickly anyway. Like when men pump and dump.
Also the best way to know is to talk about her past, if she refuses to talk about her past take that as a giant red flag, be warned that if you actually like her at all you're not gonna wanna hear anything other than she's a virgin. So careful buddy, don't get hurt asking her about all of the other guys shes been banging.
You'll know if she's a slut if she's banged a disproportionate amount of people to her age (like an 18 year old with half a dozen people she banged.) Or if the circumstances are all questionable(lots of first night lays, lots of people in relationships or while she's in a relationship, banging friends and relatives of people she knows without their permission, etc.)