BillStruhar

The proof is in the pudding... er... axils. Take a close look at the buds in the axils of the leaf stems of good leaves and bad leaves. Those of the good leaves are hard, shiny, probably reddish in color, are healthy. Those that are dull brownish are suspect. A good sign is leaves which "die", and fall off on their own. Leaves which get ugly and hang on the tree for a long time are a sign that the leaf has NOT completed its mission and been terminated by the tree, but has died of other causes. Normally, leaves do not die suddenly, they "die" of "starvation" or desiccation when the tree roots stop sending nutriments to the leaf to convert into root food, usually as a result of the leaf failing to process a surplus of food, either because the leaf has reached the end of its useful life "...best if used by... autumn" in the case of normally deciduous trees, or whatever the normal life span is for a given genera, often 12 months to 60 months for many kinds of "evergreen" trees.
Anyway, if the buds look good, DO NOT remove the ugly leaves. Sometimes, the cause of death of a given leaf is not a tree disease or mechanical damage problem, but is a leaf problem. Many kinds of trees do not shed their “dead” leaves until spring, some Oaks and Beeches among others. The “dead” leaves protect buds in the axils and are shed after those buds are mature and/or able to proceed to the next stage of life for them- expanding in spring. If the buds look good, assume the leaves hanging on are useful to the tree.

As an aside, all of the above pertains to leaf reduction via denuding (leaf trimming) too, and is why you cut the leaf at the petiole and leave a generous petiole stub which protects the immature bud in the axil until it’s darn good and ready to expand.