So How Do I Know When the Gluten is Properly Developed?

In the early stages, dough tends to be shaggy and sticky, but as it develops it becomes smooth and glossy, with the sticky feeling turning to something that can be described as silky.

An instruction found in some baking books to give an indication of dough development is to do the gluten window or windowpane test. The idea is you knead or work the dough until you can stretch a small section of it out until it is a thin film that you can see light through. This is easiest with a dough made from white wheat strong (bread) flour, harder with higher extraction (i.e. browner), weaker (e.g. plain/all-purpose) or non-wheat flours. A word of caution: a gluten window might not help you to distinguish between a dough that is developed and one that is overdeveloped.