Trees can always use some type of fertilizer to ensure better health. It also generally takes a trained eye to determine which trees are in the most need. However, you can also look yourself to try to determine which of your trees are in the most need. For example, one tell-tale sign is if the new twig growth over the past year is less than two inches, this is the ideal candidate for fertilization.
However, if it has been several years since your trees have been fed or you notice that ongoing regular growth is stunted or at a standstill, then it is most certainly time. Although they do sell many products intended for property owners to take care of the task themselves, the results will not be as effective. It is all about having the best products possible for the type of trees and even the type of climate, but also about how to properly take care of this task in order to have effective results.