The first thing to consider is the age of the roof. If the roof is over twenty years it may be a good idea to have it looked at. Check the appearance of the shingles. Are they curling up or are there a lot of missing shingles or granules? You should use a pair of binoculars to inspect the roof. We do not advise climbing on your roof if you are not comfortable, gravity never sleeps and it only takes an instant to find yourself in trouble.
When you pose this question, you are, in effect, asking if the contractor is licensed by your state and/or city. Not all states require contractors to be licensed. If your state does license contractors, then he might have had to pass a written examination in his specialty, although few licenses make this a requirement. A number of cities also require professional licensing. Check with your local licensing authority for details.
Needless to say, longer is usually better. Under three years may signal an unstable business or one low on the learning curve. On the other hand, everybody has to start somewhere. References will be helpful to doublecheck any business, and are especially important when dealing with a new business. A newer business may have a great future but it is only reasonable to be more careful when considering its referrals. The failure rate of small businesses in the first three years is very high.