I agree with Roy that it's all about track record and reputation.
Has the builder been around forawhile or is the builder a fly by nighter? A lot of people call themselves builders today but do they have the infrastructure needed to be around tomorrow- when there are problems because there will be problems.
It's important that the builder "listen" to what the client wants but then interjects with the builder's experience on what works and what doesn't based on their experience.
It's important that the homeowner, builder and architect plan out the renovation to avoid as few changes as possible because changes can be expensive. This will avoid costs getting out of control
The builder should have a reputation for building a sound structure and paying attention to the mechanical components (the guts) to avoid problems in the future.
Do they have consistent trades people whom they can hold accountable?
A good builder does not skimp on finishes or move on to another job before it is completely finished.
Can you reach them after the construction because there will be problems. So what is their after sale service?