There are generally 3 parts to making sure superior workmanship is done. First is to get an experienced roofing company with experienced installers. Second is to have a knowledgeable independent party check the work as it is happening. Third is to have an independent party check the work after it is finished. Doing the due diligence on the roofer will mostly result in the first and obtaining a manufacturer’s warranty will take care of the third. Finding an independent party to check the work as it is happening is more costly and rarely happens.
How can you tell whether a contractor does quality work that will last decades is a tough question. All of the usual investigative methods will apply – how long in business, referrals, what have you done similar to our project, etc.. Some additional thoughts – roofing materials manufacturers license contractors to install their products. Manufacturers will only warranty roofs installed by their licensed contractors. As time goes on manufacturers keep track of calls made against those warranties. Some manufacturers measure that quality and quantify with awards. Usually the awards are only publicized if the contractor puts on a certain amount of their product. So a call directly to the manufacturer of choice can yield some interesting referrals, especially if you ask the correct questions – like who has the best experience in the last 5 years / 10 years that is within 50 miles of my property? How many installations would that encompass? Coming at it from the other direction would be to find out what manufacturer a contractor is quoting, then call up that manufacturer for their recorded experience with that contractor. If you can skip any local manufacturer’s representatives and go straight to the main offices you might even get a more data driven answer as opposed to someone’s judgement (which may also be useful).
Another item to check is the experience modification rating which can point towards the safety experience of the company. A number of 1 or below is good. In Massachusetts you can find that number on-line (https://www.wcribma.org/mass/Products/ExpRatingSrch/searchratings.asp). Other states may also provide this – the contractor should also be able to provide this on his insurance agent’s letterhead.