Read in this articles about reviews!
This is going to mean they have a wealth of experience which is very important as on the job knowledge is not able to be taught in any course or book. If you think about finding some services you need to read reivews at the web such as students do finding college essay writing services where writers can say to you something about "write my essay" and where students can read different articles about how to write an essay or coursework for fast.
Using a company that has been in business for a few years also means there is a better chance that they are reliable and shall do a good job.
The theory behind that is that there is reasonable chance that a bad company shall be out of business quite quickly or you should at least be able to find out bad reports about them before you hire anybody. This is why you should also try looking at local review based websites in order to see what locals are saying about different plumbers.