How do I know which guttering contractor to choose?

Make sure the contractor you hire is licensed and insured. Protect yourself from possible liability if an accident occurs on your property.
Make sure the contractor is local. Many out of town companies will come to town, particularly after major storms, and claim to be local contractors.
Choose a company that provides some type of workmanship warranty.
Ensure that the contractor provides you with a professionally written contract that accurately describes the scope of work to be done.