Like any industry, there are good service providers, and not-so-good ones. It can be difficult to select the right people, but here are a few tips:
Look for companies that employ ISA Certified Arborists. ISA stands for International Society of Arboriculture. There is a fairly rigorous knowledge test one must pass before obtaining ISA Certification. While it is not a guarantee of excellence, it does strongly suggest that a good base level of tree care knowledge exists in the person holding the Certification. Note that individuals are certified, not companies. Find out how involved the Certified Arborist will be in the actual work being proposed. You can find a listing of local ISA Certified Arborists by going to the ISA website and searching by location. Not all local Certified Arborists are for hire, some work in the public sector.
Ask for references, and then contact them to see what the previous clients’ experiences with that company were like. Services like Angie’s List may also be helpful.
Do not believe companies making claims that they can cure any and all tree diseases. Even those of us with a lot of training and education know relatively little about the tree/disease interaction. Miracle rescue chemistry doesn’t exist. The single most important chemical you can provide to your tree is H2O, also known as water.