According to Basil the "treecoligist", a tree that's sick or dying does have certain signs to look for before calling local tree services to cut the tree down, thinking it is already dead.
For instance, he says the canopy consists of the spread of branches and their leaves at the top. If it's summer, yet the leaves are brown and dried up, your tree may be dying.
A healthy tree will be green.
Check around the trunk for rotten areas, decaying wood, or new cracks and hollow places that were not there previously.
Many trees, when sick or dying, show it in the roots. They may be lifted up from the ground. Trees do have roots that grow along the ground, but if they are lifting up and not attached, the tree is sick or about to die and topple over. If there are a lot of mushrooms growing, this means they could be feeding off dead or rotten roots underground.
Even dead trees provide homes for animals, so Basil suggests only calling tree services for removal if the tree is at risk of falling and causing damage to people or property.