One way is just to listen for it... Phase problems can make your audio sound thin or tonally uneven (certain frequencies might sound like they're repeatedly dropping out and coming back in). You can sometimes hear them more easily by switching the signal to mono, since stereo phase problems will cause some sounds to get much quieter, or even disappear, in mono mixes.
Another way to spot them is by using a phase analysis plugin. Waves PAZ Position is one such plugin which does a really good job, and some multi-purpose plugins (like Izotope's Ozone) have a different kind of phase meter, which leans to one side more often as phase problems occur. There are probably cheaper or free plugins out there that show phase problems too, I just don't know any offhand...