Licensed, Bonded, and Insured

The contractor should be licensed by the appropriate agency in your state, county, or city to do business. They should also carry all necessary insurance and bonds to cover any damages as required by your state.

References

Good contractors should be able to produce references from past projects. If not home owner names or testimonials, at least addresses of projects in your area so that you can at least drive by and see their work.

Experience and Continuing Education

Contractors should have experience installing the siding of your choice. They should also pursue continuing education (e.g.: manufacturer certifications, industry trade association memberships, etc.)

Get It in Writing

To protect yourself insist that you get the contractor to put your project in writing and make sure that it includes dates, materials, price, etc. Make sure that the contract is signed by both parties.

Use a Contractor Referral Service

These services do a lot of the background checking and other work for you. They may even arrange for a free, no obligation quote from the contractor.