In our area of the state, peach trees have set a very large fruit load. In one of my earlier posts I discussed the need for thinning to reduce the stress on the tree and to create larger fruit. The tree will also take some measures on its own to reduce the crop load.
When the tree starts to drop fruit in the spring of the year it is often called May drop or June drop, depending on where you live. The dropped fruit usually were not pollinated, and therefore do not have a viable embryo. Pollinated peaches will normally be much larger then the fruit that was not pollinated. Another way to tell is to cut the fruit open and look at the embryo. The picture shows a pollinated fruit on the left and a peach without an embryo on the right.
Some people will wait until after May/June drop to see how many fruit are left to thin off. This strategy may work, in some years where you might not think that you have a full crop left. Remember, the earlier you thin the greater fruit growth response you will get, so do not wait too long.