Only a few states recognize common law marriage. And to be married in those states you must do more than just live together; you must intend to be married.

When judges get involved in figuring out the intimate issue of whether there was an intent to be married, they look at objective facts, such as whether a couple used the same last name, filed a joint tax return, and told friends and business associates that they were married.

Even if you live in one of the states that allow common law marriage, if your fiancé and his ex didn't do these things, then they were never married and you two can tie the knot any time. If they did, he needs a divorce before you two can legally marry.