DUE DILIGENCE for contractors-what to reserach and what to ask?

[Joseph Karbus]

Hi-I'm looking for a roofing contractor to perform some roof repairs on my existing shingle roof and low slope flat patio roof. I received estimates from several roofing companies but want to perform my "due diligence"to evaluate their credibility, experience, skill, and reputation.

Beside checking the contractor state license board for complaints and the local Better Business Bureau for complaints, are there additional actions that I can take to discover which roofing contractor will perform the repairs properly and only those needed? 

I am suspicious checking "references" because the contractor might only provide those for satisfied customers and not for jobs with unhappy customers.

Thank you!