Try to pin him down. There's a fine line between being insistently curious and playing district attorney, but if you feel your significant other is evading a pointed question, don't let him escape too easily. If he tries to change the subject, change it right back with something like, "I'm sorry, but this is a topic that really interests me. WHY is there a warrant out for your arrest in New Mexico?"
Read his body language. Despite what you see in movies, there isn't one fail-safe way to examine a person's facial expression and determine if he's lying (if there were, the world would have a lot more poker millionaires). A pathological liar has likely learned to keep his nervous smiles, rapid blinking and odd grimaces under control, but if the guy you're with is an amateur fibber, he may well trip himself up.
Check the facts. In a normal relationship, a guy will say "I'm sorry I'm so late, traffic was really bad on I-95," and the gal will shrug and continue eating her salad. If that still, small voice tells you something is awry, there are resources on the Web that will tell you if the traffic was really as bad as he said it was (or if the subway was delayed, or if there was a bomb scare at the airport).
Decide what you're willing to put up with. There's not a person in the world who hasn't told a little white lie, but very few do so multiple times a day. If your beau's lies are relatively harmless, you can conceivably chalk them up to a personality quirk. Be aware, though, that folks who constantly tell little white lies have a way of dropping the occasional Big Black Lie and hoping no one notices.
Get out while the getting is good. It's possible, after years of therapy and the occasional electroshock treatment, for a habitual liar to return to the straight and narrow. But in the vast majority of cases, the rule is simple: Once a liar, always a liar. If you actually think you can "change" your guy and his lying ways, stop reading this article right now and book an appearance on the Dr. Phil show.