I get asked this question a lot. My answer is always the same:
Ask the photographer for a test shot. We are in the age of digital, and you can get instant results of the test shots on the monitor screen. Just do the test shoot right there when you first meet up with him. That saves everybody time and is a reasonable request the pg will accept.
However, if the pg feels offended by your suggestion, simply walk away. You shouldn't consider such a guy for yr photoshoot in any case. Photography is all about people relations.
I won't go into details of how you should evaluate the test shots on the screen, but if you need further details, pm me.
cheers