From a distance
- The tree leans, or leans more than it used to
- Dead branches in the crown
- Sections of the canopy with no leaves, or thin leaves overall
At the base
- Soil that is raised, heaving or cracked on one side
- Fungus or conks on the roots or trunk (mushrooms in mulch or rotting wood in the soil are normal)
- Deep hollows near the ground
- Sawdust, which can mean carpenter ants or wood-boring insects. Carpenter ants nest in dead wood.
On the trunk and limbs
- Cracks, splits or cavities
- Missing bark
- Two or more trunks joined in a tight V. The joint can split, and should be looked at right away if a crack has started.
- Dead limbs hanging in the tree after a storm
What to do
If you see these signs on a tree that could hit the house, a driveway, a street or anywhere people gather, have it checked by a certified arborist or an experienced tree company. A split trunk should be dealt with immediately. Catching problems early is cheaper and safer than a removal after the tree fails.
Sources
General information from public sources. Show Me Tree Pros is a directory, not a tree company; for a specific tree, have a professional look at it.