When fruit is dropping and birds are feeding in the tree, chances are you're approaching harvest time for that crop. Then apply the taste test. These are perhaps the easiest ways to determine harvest time in the School Orchard, with its multiple varieties of trees.
B - "More in Depth"
For the Apples, you should be able to indent the skin and flesh with your finger nail, and when you cut it open the seeds should be dark brown or black. If seeds are still light color, you're not quite ready for harvest. If there is good sunlight on the apple, and the nights are cold, some red color will still develop.