Selecting the best head shot photographer for your needs shouldn't be a quick process. Dig through head shot portfolios and look for consistency within the work. Then narrow down which photographer produces head shots that are in line with the looks you need.
To make this process easier I recently made a change to my head shot samples where I've separated the different styles that I shoot. The Modern (shown above), Corporate, Traditional, etc. When reviewing photographers' work, pay attention to expression and postures. Look at the details such as where the hair lays, the creases in the shirt, position of the collar, tension in the neck... that list goes on and on. All of those little details should NOT be random and they will make or break a good head shot.
Lastly, look at wardrobe. Colors of shirts, patterns, neck lines, layers; talk to your photographer about what photographs well and what should be avoided so that you have everything you need when you show up for your head shot session.