You wouldn't hire a baker without trying a sample (or 12) of her cakes, would you? This might seem obvious, but many brides take it for granted that a photographer with an established business must ooze talent. Sometimes, that's not the case.
Most upstanding photographers publish digital portfolios online, which will help you weed out the less-than-stellar candidates. After you've chosen a selection of potential photogs, set up interviews to see the prints in person. Be sure to take into consideration the diversity of their shots, their use of lighting and how comfortable the photos' subjects appear. A truly talented photographer can get even the stiffest brides to loosen up for the camera.