The windowpane test is one of the best ways to tell if you've sufficiently kneaded your bread dough, though it can sound like a pretty bizarre instruction when you come across it in a recipe! Here's what you do...First, cut off a small piece of the dough about the size of a golf ball. Hold it between your thumb and first two fingers like this:
Next, gently spread your fingers and thumbs apart, stretching the dough into a thin translucent membrane (ie, a windowpane), like this:
If you can stretch the dough without it breaking, that means the gluten is well-developed and your dough is ready to rise. Pat the ball back into the larger batch and you're good to go!
If the dough tears before you've fully extended your fingers, the gluten isn't quite ready yet. Knead the dough for another two minutes and try the windowpane test again.