I follow forums such as this one and soak in information about respected outfitters. I give more weight to the comments from long-time respected forum members than those from members that i know nothing about.
I also reach out to certain trusted forum members to learn more and pick their brain. After finding a few good potential outfitters, i search the web very carefully to see if there is any reliable negative information about the outfitter. If they pass this phase, i reach out to the outfitter and get more information from the outfitter. It's amazing how much you can learn about a guy from just hearing how he reacts on the phone. Does he come off like a guy that over promises and almost guarantees a book animal or does he sound like an honest guy. While not as helpful, i also call successful and unsuccessful references.
While i'm not batting 100 percent, i have a good track record using the above approach. This includes finding and hiring several outfitters from this site.
Good luck.