#5 . The Windowpane Test
The purpose of kneading is to develop gluten. Gluten is a protein formed when two other proteins, glutenin, and gliadin, combine with water and then get agitated--stirred, mixed, or kneaded. (Incidentally, that's why when you make some baked goods, you mix minimally and gently once you add liquid to the flour. You don't want much gluten to form in the case of cakes, pancakes, and muffins).
But how do you know when enough gluten has formed to make your bread rise and get a lovely chew? It's called the windowpane test. After kneading for several minutes, tear off a small piece of dough (if it stretches a lot before pulling away, that is another good indicator of good gluten formation).
Roll the dough into a small ball and flatten it into a disc. Start rotating and stretching the dough as if making a tiny pizza. You should be able to get the dough thin enough that it gets nice and translucent before tearing. If the dough tears before stretching out nice and thin, you know you have more kneading.
This test works best on white bread as the sharp edges of bran in whole wheat and other whole grains tend to cut some gluten strands. That's why whole-grain bread tends not to rise as high as white bread. You should still be able to stretch the dough into a windowpane, but you won't be able to get the dough as thin.