Most bread recipes, mine included, will tell you to keep kneading until the dough is smooth and elastic. That's simple enough, right? The dough goes from being a clumpy, lumpy mass to something that is cohesive and, well, smooth. And instead of tearing off in ragged chunks, it begins to stretch. And the more you knead, the stretchier it gets.
But when do you stop kneading? How elastic does it need to be?
Oh, if only there was a simple test for checking the elasticity of the dough!
But wait! There is! It's the Windowpane Test!
Ask about the windowpane test in any serious bread-baking forum and it's almost guaranteed that some wise guy will tell you to throw a hunk of dough at the window, and if it sticks, it's done. In kinder, gentler groups, someone else will probably explain the windowpane test.
But pictures are so much better than words, aren't they? To begin the windowpane test, pull off a small bit of dough, about the size of a marble. Flatten it out, then begin gently stretching it.
Keep stretching. Imagine you're making a pizza for Barbie, and she likes the super-thin crust.
If you can keep stretching an pulling the dough until it's a thin membrane that you can see through, the dough is elastic enough.
Here's a closeup of fingers seen through the dough. You can sort of see the webby network of gluten.
You can keep stretching and eventually it will tear because it's simply too thin or you've poked a finger through. But it should be able to stretch very, very thin before it breaks.
Doughs that include seeds or are made with gritty flours will tear sooner, simply because the bits tear through the membrane. This is the same thing that happens in the bread as it bakes, and it's one reason why breads made from whole grain flours tend not to rise as well as those made from more refined flours. But even with those doughs, you should be able to stretch it well enough to see that thin membrane.
And if your dough passes the windowpane test, the gluten is developed well enough for it to hold the bubbles while it rises, and stretch even more as the gasses expand from the heat. Simple, hmmmm?