The internet is a great thing, and with forums like this, or even just popping into a Google search, you can find out a hell of a lot about people. My advice is to always:
-Check Reputations and availability. Some guys do this fulltime and some do it on the side and have limited slots they can accomodate. Obviously each option will have an effect on how much time they can dedicate to you.
-Check Price Ranges. If someone is doing this as a sole source of income, they might have a greater need to charge more than someone who relies on their coaching for 'gravy' and can work with a sliding scale. Your own situtation will also factor in, as a college student might be shocked at a quote that a company CEO might drop for lunch.
-Check that they have a history working with people with the goals you have (weekend warrior, Physique, BBing etc). If for example you're concerned with changing your physique, but like to engage in athletic activities as the same time, this might throw a curve ball at some people who can shred you down, but totally kill your day to day performance in the process. Always ask questions!