You should have a reasonable policy when deciding what to do about a tree that is hollow inside the trunk. Because the defect is on the inside, it is harder to see and judge the danger from the outside. A reasonable policy can t be based on emotion. Remove a tree with a large cavity when it has a stationary target like a house.
Different species have different mechanical strengths and decay strategies.
A declining or leaning hollow tree is much riskier than a vigorous and upright one. Both are good techniques for finding the largest cavity, and to get a rough idea of its size. Measure how far up the bit you are when resistance drops, the shavings become discolored, or nothing more comes out. You can try to slow its decline, but you can t stop the decay. Fertilization will usually not help much, although aerating the soil to promote root growth is nearly always beneficial to older trees.