I have written some important information about roofing work on my web site, which is unbiased info, given that I do not sell roofing work, working as a Gutter Contractor for the last 25 years:
http://dmr-gutters.com/rf/roof...

For a start you can ask them if they use only aluminum or copper flashing. No steel, which will rust out in less time than the lowest life roof shingle! This will be very telling of their commitment to quality. If they do not know this off the top of their head that is highly suspect, but if they really are that dim you can ask them to test it with a magnet. Only steel or low grade stainless steel will attract a magnet. Aluminum flashing does not actually cost more and cannot rust. Think of how long they have been making sturdy aluminum ladders with no protective paint coating needed. So don't fall for the lie that aluminum is not sturdy enough to use. It is just hard for them to get, since all the Roofing Suppliers will not stock aluminum flashing parts for the single reason: if they can compel all Roofers to use only steel flashing they will then never have to pay-out on a warranty claim.

Also, ask them about reflashing around chimneys and dormers, which is the most neglected area of any roofing work. Ask to see close up photos of their work in those areas. Such as shown here:
http://copper-by-design.com/rf...

Another very important detail that will void the warranty of the roof shingles is if they use a staple gun. They should only be using a coil nailer with stainless steel roofing nails. Sure those will cost them 5X more, but in the whole roofing contract cost this is less than a 5% increase to the end user.