What You must Always Do
I've made many mistakes and I've learned from a few. I also try to pick up lessons by watching other people. I feel like I should tip them afterward, when I'm lucky enough to see a real lesson in action. Remembering to do the following things helped simplify my life tremendously. Think of them as affirmations, if you're into that sort of thing.
Believe your partner
It's possible they're lying, but it's also possible they're telling the truth. It's safest to decide they're telling the truth. If they are, you'll be a saint. If they aren't, they'll catch themselves in the lie later, or feel really bad and possibly even apologize. Maybe buy you a milkshake.
If you're absolutely sure you're being lied to (I mean, really sure, as in photographs and signed testimonials), then ask your partner to explain the discrepancies. The important thing here is that you're still willing to listen to them, to hear and understand what they know.
If you find you can't depend on your partner, that you simply can't believe them, or that believing them results in trouble for you, then your trouble is with that partner -- not with the act of believing.
Be Sincere
If you are sincere when you talk or otherwise deal with your partner, it becomes easier to accept their sincerity.
Apologize When You Screw Up
It doesn't have to be elaborate, but if it's not sincere, don't bother -- you probably won't fool anyone.
To combine the above two things, when your partner apologizes, accept it. You might be incapable of knowing whether they're sincere or not. In which direction would you like to make a mistake?
Tell the Truth
You might be surprised at where it gets you. Here are some excellent reasons to tell the truth:
If you don't want to tell them anything, consider doing the courtesy of explaining why. You don't have to, but you might discover something new right then and there.
Wipe Front to Back
Always.