Ask to see the contractors license and certificate of liability insurance.
Ask if the new roof has a warranty?
What does it cover (labor & workmanship or just the shingles)?
Does the manufacturer require felt paper to be put on the decking before putting on the new shingles?
Is there enough attic ventilation?
What are the manufacturer specifications for the roof that your contractor is considering?
Do you have all the specifics ?
Ask the contractor to leave behind a bundle of shingles just in case you encounter a wind damage, you will then have replacement shingles that match your existing roof.