You should always check with your local Better Business Bureau (BBB) for people with grievences.
Does anyone know if plumbers are licensed? If yes, make sure the person has one. My father is a lawyer and I've learned a few things by listening over the years (don't tell him, he might send me a bill):
1. Never pay up front. There have been a lot of people who get rooked with half-done work or worse and the plumber refuses to finish.
2. Never sign anything until you are satisfied with the work.
3. It is best to put in writing (like a contract of sorts) what is expected before the job starts, get a legitimate estimate on paper (not just some verbal yada yada)
4. If the person is charging you for more than what was on the estimate and agreed upon, there must be some sort of explanation and receipts for parts.
You get the idea. I know this sounds like a lot of work, but it has saved a lot of people a lot of money in the long run. I have to say that I've only had one person refuse to these terms and later learned they were a "fly by night" type of contractor.