Make Sure They Have A Tax ID Number
A business big enough to handle commercial jobs will have a tax ID number. They should be willing to disclose this information when you ask.
Ask to see their business license. Any legitimate business doing business anywhere needs to have a business license from the town they are doing business in. It should be valid for the current year.
Industry Association Member
If they are a member of the local industry association, it is a sign that they want to stay on top of industry news. They will also be made aware of any issues that may arise.
Insurance And Bonding
For both their safety and yours, they should have their own insurance and bonding. This protects them in case of accidents and protects you in case of theft or anything else that may happen while the workers are on your property.
Ask what safety standards they have in place. Their employees should have had safety training to ensure safety while on the job.
In some locales, roofers are required to be licensed. If this is so in your area, make sure they are licensed. If they are following the law, then they will also follow your needs.
Ask for references if they do not offer them to you at your initial meeting. Ideally, they should offer you references without asking.
A portfolio of past work is also a sign of a reliable company that performs good work.
If they are a public company, you can research their financial statements. If you ask them, they should be able to give you this information too.