Four signs matter most: mushrooms or shelf fungus at the base, which usually means the roots are already rotting; a lean that is new, especially with soil heaving on the opposite side; large dead limbs in the upper canopy; and a crack where two main stems meet. Any one of those on a tree near a house is worth a professional look. A tree that has visibly moved is urgent.
Fungal conks at the base
Mushrooms or hard shelf fungus at the root flare almost always mean the roots and lower trunk are decaying. The canopy can look perfectly healthy while this is happening, which is what makes it dangerous. A tree can be structurally hollow and still fully leafed out. This is the single sign we treat most seriously.
A new lean, and what the soil is doing
Trees that have leaned their whole lives are usually fine. What matters is change. Look at the ground on the side opposite the lean: if the soil is cracked, heaved or lifting, the root plate is failing and the tree is in the process of coming over. That is not something to watch and see about. Sandy Tampa-area soil holds a root plate less firmly than heavier ground, and a run of heavy summer rain is often what starts it.
Dead limbs in the canopy
Laurel oaks in particular are prone to shedding large deadwood, and they are everywhere in the older Tampa and Brandon neighbourhoods. A laurel oak in its sixties can be hollowing from the inside while looking fine from the street. If you can see bare grey limbs above the leaf line, they will come down eventually, and the question is only whether anything is underneath.
Cracked unions and included bark
Where two main stems fork, look for a crack running down from the union or a seam of bark pinched between them. Those are the joints that split in a storm, usually taking half the tree with them. It is often fixable with reduction pruning or a cable if it is caught early, which is a great deal cheaper than a removal plus a roof.
What does not necessarily mean the tree is dying
Some leaf drop outside autumn, a few small dead twigs, lichen on the bark, or surface roots at the lawn. Lichen in particular alarms people and is harmless. A palm losing its lowest fronds is doing what palms do. Not everything that looks wrong is a hazard, and we would rather prune a tree you like than sell you a removal.