Be sure to check out the photographer's online galleries to see if you like his/her style. Does it look like quality work? Compare his/her to other photographers to weed out the inferior work.
Photographers also have different styles, so no two good photographers are going to give you images that look the same. This is not to say that one style is better than another, but one style may better fit what you are looking for in a portrait.
Use a photographer that can deliver what you want.