Visit a job in progress, if possible, and see the conditions of the site. Talk to Sub-contractors and suppliers used by the contractor. Call the Better Business Bureau, local building associations, or the local building office and see if any of the contractors have had any problems. Talk to the contractors themselves. If there seems to be a problem then don't hire them.
Good communication is very important.
You should have a top two or three by now. Take bids You need to know as many of the details concerning the project as possible to get accurate bids. Ask for itemized bids. The more detail on the bid the better off you are. Make sure that the contractors are all bidding the same work to be done.