First, verify that the Contractor is licensed. Florida Law requires that a General Contractor or Home Improvement Contractor be licensed.
Verify that the Contractor is insured. State Law requires that they carry workers compensation insurance. Ask to see a certificate of insurance. You may also want to ask for references and be sure to check them all out.
Finally make sure all agreements are in writing. Be sure to reduce all change orders, or contract amendments to writing. Make sure the Contractor obtains all required building permits and pass the required code inspections. Before making the final payment be sure to obtain a certificate of completion or certificate of occupancy issued by the Building Division.