Realize your photographer is among one of the only people that will be with you the entire day... make sure that you like him or her.
When meeting with a photographer, expect to see a portfolio of a wedding in full. This way, you're not seeing just the best images from a collection of different weddings.
When meeting with a studio, make sure you meet with the actual photographer not a sales rep. Make sure it's their work you are looking at.
Ask for past customer references and be sure to call them.