They should respond to your calls and your emails. They should answer all of your questions to your satisfaction. Communication is key to every successful project.
You want to work with a company that is of a professional level. This includes the people in the office, project managers in the field, the ownership, and all of the contractors.
They should provide you with valuable construction expertise and insight. You will also want to have a general estimate of what to expect for costs before you even start with the architect.
Does the builder listen to you and share your vision?
How flexible is their process, and when and how can you make changes along the way? Production Builders are the type that won't let you do any customization or if so very little, that's not what you want. Nothing compares to a true custom home building experience!
Quality - Be sure to look at some of their homes you can can see and feel the quality they offer.
References - yes you can see the company reviews online on facebook, google, and Houzz. You should also visit one of their jobsites and talk with a few of the subcontractors. Trades can give you the best, behind the scenes information. How the builder treats others, quality management, how involved the builder is, how long they have worked for the builder, and so forth.