Cavities
Holes in a tree could mean the structural integrity has been compromised, and a bigger hole usually represents a larger tree health issue. Any cavity needs to be inspected by an experienced professional.
Discoloration
While trees are never uniform in color, obvious discoloration on the trunk or limbs could be an early warning sign of structural problems or poor tree health. It's a good idea to have a professional take a look to determine.
Inclusions
Inclusions are when a tree forks and the limbs begin to grow back into each other. Branches with inclusions have a weaker connection to the trunk. Cabling, bracing and pruning are often used to prevent tree failure in instances of inclusion.
Lean
A tree that leans predominantly in one direction is at a higher risk of failure, because it is only being supported by one quarter of the root system. Leaning trees can be protected from failure through weight reduction by pruning, which can adjust the weight-to-lean ratio.
Dead Wood
Dead wood in the upper canopy of a tree can be the result of wind damage or something potentially more serious like a large trunk cavity, underground root degeneration, or generally bad tree health. Dead wood should only be removed by a licensed professional.
Parasites
Fungi or sucker growth can be harmful to tree health. Spanish moss and mistletoe, while not parasitic, can also choke out a tree's foliage if not kept in check. A tree care expert can identify problematic parasites and select a safe course of action for the tree and the property owner.