You don't. It's a matter of trust. It also depends on your worldview.
Some people are naturally suspicious and won't fully trust anyone until the person proves himself trustworthy. Others prefer to take people at their word until they give you a reason to stop. I fall into the latter category, but neither approach is right or wrong. The best thing you can do is follow your gut--and his track record. If he has lied to you before, no one can blame you for being suspicious of everything he says now. That's how trust works: it takes a long time to build and only a second to destroy.
Thanks for the question.
I trusted my boyfriend up until the day he left me for my roommate.
Relationships should be built on trust, but don't allow yourself to be blinded by your affections.
If there were a magical, foolproof way to ensure that every significant other in the world could be trusted on their word, this website wouldn't exist and many more of us would probably be in jail for acting on the knowledge that we couldn't trust the person!
I believe you should trust a person until he or she gives you a reason not to. It's a wonderful thing to be able to sleep cuddled up against your partner and think, "If they wanted to, they could probably bang half the country. If they wanted to, they could talk bad about me to all their friends. If they wanted to, they could divulge every little bit of our sex life to the internet, including videos. But they don't want to, because they love me."
i agree with Sarah. if Cary, or anyone else for that matter, had the final answer on this one he would have bottled it up for sale and made a killing off it. trust is a gamble, no doubt. sometimes the return on the investment is incredible. and sometimes it leads to big losses. the hardest lesson to learn is that just because you get losses by playing with one person, doesn't mean you will with the next.