Trees located in public spaces, such as parks and street rights-of-way, are typically the responsibility of the municipality. If this is the case, consult with the local department of parks and recreation or public works. These departments often have an arborist (a tree care professional) on staff or through contract who should be qualified to provide tree advice. If the tree is within or near a utility easement, you may want to consult with the utility service provider. They also typically have tree care professionals on staff or through contract.
For trees on your property, you may seek advice from your county's local Cooperative Extension Service office. Your community may also have a civic group or non-profit agency available to assist with a range of tree-related issues and information. These groups can often be contacted through the Alliance for Community Trees or your local Extension office.