Go out and look at spec homes that the builder has worked on. Look very closely at all the small details in craftsmanship. Try to look at houses that don't have furniture in them and are not staged, this will allow you to focus on the house itself, rather than the interior decorations and design.
Don't go with the cheapest custom home builder, especially if there is great difference in price. Your home is likely to be your most important purchase, and you don't want to be cutting corners and using inferior materials. If the builder can offer cost saving that is terrific, but recommending poor quality to save money is not a good idea.