Anyone can cheat under the right circumstances. That's why it's so common for people to cheat when they're on vacation somewhere. Romantic atmosphere, "exotic" partner, low risk of being caught or a sticky situation afterwards...
I agree with those who said that the people who are less likely to cheat are the ones who realize their weaknesses, and stay out of situations where they'd be weak. For instance, they don't start eating lunch alone with an attractive co-worker. The only way you really know if someone makes good decisions is time spent observing them. You can also make cheating less likely by giving her frequent orgasms.
In the end, this is really a useless question to ask. By the time you've agreed to be monogamous with her, you'll already have decided she is trustworthy. If you haven't, you're a fool for agreeing.