Lemon trees, native to the warm climates of Asia, are now grown throughout the world. These trees require warm temperatures throughout the year, as a single night of 20-degree temperatures is enough to permanently damage them. Lemon trees, however, go through a "sleeping" period during which they appear to be dead. For this reason, it isn't always obvious whether a lemon tree is dead or simply in its dormant stage.
Examine the leaves on the lemon tree branches. If the leaves are wilted, brown or nonexistent, the tree may be dead. However, this could also mean the tree is in its dormant stage.
Break a small branch or twig off the tree. If the branch breaks off without bending and appears to be entirely brown in the center, the tree may be dead.
Scrape the surface of the bark on the trunk with your fingernail or a knife. If the flesh of the tree is brown and dry, the tree is likely dead.