Window Pane Test
From The Bread Baker's Apprentice by Peter Reinhart (p. 58)
The most reliable method to determine when gluten development is sufficient is called the windowpane test, sometimes referred to as the membrane test. This is performed by cutting off a small piece of dough from the larger batch and gently stretching, pulling and turning it to see if it will hold a paper-thin, translucent membrane. If the dough falls apart before it makes this windowpane, continue mixing for another minute or two and test again. It is very difficult to overmix bread dough, that is, to overwork the gluten to the point of breaking the molecular bond.