Ask questions and get answers from experienced industry professionals
Look to see if they belong to any local organizations such as NARI.
What type of fence are you leaning toward? Once you get into the larger fences, they are typically 4 or 3 board style horse fences.
I would definitely look at reviews on Angie's List and GuildQuality. With these any contractor should be licensed and fully insured. We are a fence contractor and we focus on customer service and high quality. Be sure to ask them how they ensure quality.