Are you inspired? - Do you really love the photographer's work? Do you like the images, the composition and the style of photography? Do the images inspire you in some way or touch your inner self? Do you feel at peace when looking at the photos or perhaps they inspire you to take some action? Either you connect in some way with the photographer and his/her work, or you don't.
100% Passion - Is the photographer a passionate individual? Passionate about life, passionate about people, and of course passionate about photography.
It's all about communication - Do you get on with the photographer? A vital ingredient of the photographer's art is that of communication and personality. Does the photographer have excellent personal skills? Does he/she inspire and motivate? Is he/she 'down to earth' and approachable? If the photographer is the type of person you could easily spend time over coffee (or wine), while enjoying discussing matters other than photography, then it's highly likely this relationship will produce superb images. Never select a photographer without meeting them in person.
Art matters - View a broad range of the photographer's work and photographic styles. Even if you need a wedding/portrait photographer, try to gauge the photographer's creative talent by also viewing their fine art prints.
Size matters - Ensure that you see a portfolio's photos printed and enlarged. It is possible to hide technical and creative faults in small web/computer-based images. Large prints will clearly differentiate the great images from the flawed ones. The best photographer's work will look even better when it is printed large.
Creativity and technique - Can the photographer demonstrate a mastery of both creative and technical skill?
Only the best is good enough - Is the photographer committed to excellence and customer service? Does he/she offer a money-back guarantee?