Tree poisoning test
Once the plant is dead it is almost impossible to determine what killed it. Diseases usually show symptoms or signs of the disease, but they are usually gone after death.
Insects may show damage, but after the fact it is hard to determine if the insect damage was before or after death.
If you had an idea of what poison was used where on the tree, the living tissue could be tested for the chemical. This is expensive and not usually done unless the list of chemicals suspected is very short. Where to test on a dead tree and the results would probably be negative unless you knew specifically what to test for and where and the poison might be gone.
Environmental stresses usually show some symptoms as well but leave little trace on a dead tree.
After the fact is is extremely hard to determine the cause of death.