#5 . The Windowpane Test
The purpose of kneading is to develop gluten. Gluten is a protein that is 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 at all in the case of cakes, pancakes and muffins).
But how do you know when enough gluten has formed to let your bread rise nice and high and to 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 then flatten it into a disc. Now start rotating and stretching the dough, as if you are making a tiny pizza. You should be able to get the dough thin enough that it is gets nice and translucent before tearing. If the dough tears before stretching out nice and thin, you know you have some more kneading to do.
This test works best on white breads as the sharp edges of bran in whole wheat and other whole grains tend to cut some of the gluten strands. That's why whole grain breads tend not to rise as high as white breads. You should still be able to stretch the dough into a windowpane, but you won't be able to get the dough as thin.