•  Does the contractor offer references you can check?
• Ask to see a portfolio
• What materials will be used?  Ask to see a sample.
• Does the contractor have insurance?
• What kind of written warranty is provided?  
• Is there a possibility of added charges?  (taxes, hard digs, etc.)
• How many rails are being used?  How are they installed?
• If picket fence, what grade will be used?  What is the thickness?

A Note From a Customer:
"I am writing to commed you for the excellent job that Jim Smith did on behalf of your company.  Not only did his crew build a beautiful, first-rate fence in a timely manner, but he also monitored their progress closely making cetain it was a quality production.  
... We Will certainly recommend your company in the future."
L. Gariepy, Sr,
Canyon Lake, TX