No matter what I'm buying, whether it's a hunt or something else, I try very hard to avoid being handed a list of hand-picked references. How many guys are going to knowingly give you contact info for someone they know was unhappy?
Asking for the contact info for the guys that have hunted say, the last 3 years for the same species, same area, same time of year and same guide, or some other list that you come uip with is a much better way to go. It removes much of the bias.
Always check regulatory records and professional organizations for guides and outfitters. It's not a deal-breaker if he doesn't belong to any organization because not all good guides and outfitters do, but it is worth finding out if there is a reason.