How to check builder's reputation?

In the United States, check Yelp or Angie's List. Elsewhere, you will need to talk to people who worked with the builder.

Some states have state boards for contractors (at least California does) and you can call them as they deal with various people reporting irregularities, and they investigate them.

I'm not disagreeing with Jespah - her suggestions are good ones.

Far too vague of a question. In what location - and for what country? If in USA, what state, region?

Good Reply....