Bread bakers hear "Knead till windowpane dough is achieved," but what does that mean and how do I check for that? Talking with someone asking this question, I decided to do a quick video of a dough I was working on and to show the tool I use to do this test.
After the initial kneading, when my dough looks pretty smooth, I stick my tool deep into the dough and slowly pull upwards. I want the dough to stretch and stretch (not tear), till it is see-through (windowpane) thinness draped across the tool. Video of this process to come but I hope the description/screenshot (hopefully you can see the navy spatula peeking under the dough) of this process and knowing the tool for the job is helpful!
The tool I use is a 7" melamine spatula/spreader but a similar wood tool would also work. It needs to be something that does not have a sharp edge, which would cut the dough, and both melamine and wood have smooth rounded edges.