Here are some helpful ways of how to pick your contractor:
a) By recommendation
b) Is he/she a member of a trade body? It is often not necessary for domestic projects, as there is no professional body that is dedicated to this market.
c) If not, has he/she a) references and b) a portfolio?
d) How long has he/she been in business and how many projects of your type have been carried out.
e) Ask technical questions