There's a number of ways, but using forums like this and either asking past customers or knowledgable others for any available 2nd, or even 3rd hand information might be one way to start.
Or you could ask the builder himself for references, i.e.- a list of previous customers that you could contact.
Or you could even ask other builders for their opinions of the builder in question if they are familiar with his work.
One of the problems about inquiring about reputation is that it doesn't always predict the future or each and every project in the past. It might only show a narrow slice of history, and it can result in information gaps which may or may not be very important to your personal project.
Also, if you ask for information in the wrong place, like a public forum where the builder is a long time member, than some dissatisfied folks might not want to complain because they may not want to create any animosity. But you could always encourage the confidential exchange of information using more private methods to communicate, such as personal threads & messages, home emailing or by phone contact.
Being able to actually meet a builder and view some of his work/workshop in person, or to see enough pictures of his recent work, should help one to form at least a basic opinion.