An arborist cannot undo an old, bad pruning wound, but he or she may be able to help your tree acquire the necessary nutrients to defend itself against progressive decay. Urban trees are frequently living with infections from old wounds. An arborist can monitor insect and disease problems and prescribe practices that can help the tree defend itself. He or she can advise you if you plan physical changes to the environment that will impact the trees and their root systems. To become a Certified Arborist, a person must demonstrate an understanding of what it takes to keep trees healthy by passing a grueling test.
Certification is no guarantee, but it is a step in the right direction.
You should insist on a signed contract. Requiring work be completed according to ANSI A300 Standards is advisable. The American National Standards Institute can help you better define how work on your trees should progress. They define techniques for pruning, fertilization, safety, and other categories of tree care. Your arborist should have a copy.