You don't/can't know if she was a slut before you met her.

You don't/can't know the last time she was fucked.

You don't/can't know if she has cheated on you or will cheat on you.

To solve this problem:

You do/can know that that all women are manipulative opportunists.

You do/can have the ability to treat her as a liar until proven otherwise (hard to do).

You do/can have the ability to treat her as if her forehead were stamped with "DANGER: VOLITLE. HANDLE WITH EXTREME CAUTION."

You can take the red pill.