I agree with the senior members above.
For the college girls I was dating, I didn't care whether they were or not, I didn't ask, sometimes they said they would be, but I figured out a way to tell [in my mind] that I think was correct:
Girls who were overall *busy* and were coming to see me 1-2x a week were likely to be monogamous.
Example -- This super-sexy girl Chloe, 19, junior at a very selective school. She's in a sorority, that's one weekend night a week gone off the bat. She's studying at the library in her room 3-5x nights a week. Oh, she also did track & field [not D1 but still]. Plus regular hanging out with female friends. And she was president of some club too.
Our relationship used the rest of her time, essentially. She had one 'free slot' left and I filled it, no pun intended. That's certainly not to say she couldn't find an hour somewhere to have sex with a new guy, but my personal experience is these well-organized girls were looking for one guy they could go off-campus and fuck 2-3x on a Saturday/Etc night and also be taken out for drinks/dinner. They're not looking for some new dick on campus, if that's what they wanted they wouldn't be using up all their spare time on a guy more than 2x her age. JMHO.
I also think smart girls realize the more they invest, the more we invest in them, and the better the relationship is. Also, some of these girls will just ghost you so they can sleep with a new guy, they're serially monog.
Another topic around this we've spoken about here is women who you can tell aren't used to being touched by men, are probably inexperienced and less likely to cheat. Your low N girls.