Learn what his or her friends are like. Birds of a feather flock together. You can learn so much about a person by the company he keeps. If he weren't of the same stripe, he wouldn't enjoy their company. It seems to me that the person who wants to cohabit doesn't possess those character qualities which are needed to make a marriage succeed.
Marriage is built on love, commitment, and trust.
Of course, change is possible. Normally speaking, however, the moral flaws which exist before marriage will also exist afterward. My advice, therefore, is don't leave a stone unturned even if you have to hire a private detective. Know the person you're going to marry. So much pain or happiness rides on that decision.