How to tell if a tree is dead is a question you can start answering yourself, but there are limits to what a ground-level check can confirm. A tree can pass a surface-level scratch test and still have significant internal decay, structural failure in the root system, or a disease that is not yet visible at the branch tips.
If your results are mixed, if the tree is large, or if it is located near your home, a power line, or a structure, the next step is a certified arborist evaluation rather than continued monitoring. Our post on how to conduct a tree health assessment covers the structural inspection layer in more detail, and our tree trimming and pruning services address trees that are declining in sections but still salvageable.
For trees that have crossed the line, professional tree removal is the safest path forward. A dead tree does not give visible warning before it fails, and waiting on a confirmed dead tree near a structure is one of the most preventable hazards on a residential property.