Ask questions and get answers from experienced industry professionals
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.
Look to see if they belong to any local organizations such as NARI.
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.