How do I know if I have a pollination problem with an existing tree?
If you have inadequate pollination, a mature tree will produce blooms and often even fruit, but they will abort and drop by the time they are about the size of a loonie.
This can be a simple matter of lack of pollination partners i.e. planting only one apple tree. Luckily, this is simply fixed by planting a pollinator nearby! If your tree was producing fruit reliably for years but then suddenly stopped, perhaps your tree was being pollinated by a neighboring tree that was taken down recently. If your tree has never produced fruit, perhaps you never knew you needed 2-3 trees of different varieties from the beginning.
If you have lots of fruit trees, the problem is more likely frost damage: a late spring frost knocking out the tree in question or its pollination partner. Stone fruits are notorious for blooming right around the last frost of the season, and that can cause a greatly reduced harvest. To mitigate this damage, try to plant sensitive trees like cherries, peaches, and apricots on the north side of a bush or building in a sheltered location: this will help the tree stay dormant longer, hopefully missing that final frost. Harnessing the power of moisture (sprinklers) and wind (turbines) can also help, but are often more of a hassle for the backyard grower.
Occasionally if a tree is very old or severely nutrient deficient, then it may struggle to product fruit too. Conversely, if a tree is too young, it may bloom but not produce fruit, or may abort the fruitlets. Generally, if a tree is 5-7 years or older, it should be bountiful.