For the windowpane test, gently stretch a small piece of rested dough between your fingers. Well-developed wheat dough can form a thin, translucent membrane before it tears. A sample that resists stretching may need a short rest, not necessarily more mixing.
The test assesses gluten development, not fermentation readiness. Whole grains, bran and different dough styles affect the result; gluten-free dough does not behave like wheat dough.
Reference: Further reading.