Most Helpful Girl
You never truly know if someone will cheat on you. In a relationship, you're supposed to put your trust in that person fully. It's important to pick wisely. Choose a partner that truly loves you and has good morals and respectable traits. Even then, hypothetically, they could still cheat at some point, but it's way less likely. But really, it depends on the individual. I would never cheat on my partner, just as he hasn't cheated on me. I know my partner will never cheat on me, ever. He's never cheated on any of his past girlfriends and he's a good person with a big heart. On top of that, he's very in love with me. I can feel how much he loves me and he's so happy with me, he doesn't even pay attention to other women, let alone do anything considered cheating. As my lifelong partner and best friend, I trust him and he's only ever thought of being with me.
Most Helpful Guy
I don't think it's possible to ever be 100% certain and I personally think it would be a mistake to pass up on a potentially good relationship out of fear of something like that. Almost nothing in life is ever risk free.
What I would look for early on though is whether she's ever cheated before and whether she's had casual sex (FWB or hookups) any time in the past decade or so. (I don't think what happened more than 10 years ago probably matters much since people change and mature over time.) Also, does she seem impulsive or irresponsible or does she tend to be more careful, cautious and responsible. My opinion is that someone impulsive and irresponsible is probably more likely to cheat (not that all would, but probably a higher percentage).
I think most people in a good solid relationship will not cheat so you can help your chances by treating her well, making her feel comfortable bringing up any issues she has, being sure she feels loved, and working to achieve a mutually agreeable compromise when you have disagreements.
It might help for the two of you to both take the quiz at www.5lovelanguages.com to find out your love languages and share the results with each other. By knowing what makes her feel loved and being sure you do it, that should help keep her happy and keep her from cheating.