Kneading for 10-12 minutes by hand or 8-10 minutes in a mixer are the general standards; if you've been massaging the dough for that length of time, you can be pretty confident that you've done your job. Here are a few other things to look for:
1. Smooth Dough - The dough will start out looking like a shaggy, lumpy mass and will gradually smooth out as you knead. By the time you finish, it should be completely smooth and slightly tacky to the touch.
2. Holds Its Shape - Lift the ball of dough in your hand and hold it in the air for a second. If it holds its ball shape, that means the gluten is tight and strong. If it sags down between your fingers, the gluten is still loosey-goosey and needs some more kneading.
3. The Poke Test - Give that ball of dough a firm poke with your finger. If the indentation fills back quickly, you're good to go. If it stays looking like a deep dimple, continue kneading.
4. The Windowpane Test - Pull off a golf-ball-sized piece of dough and stretch it into a thin sheet between your fingers (as pictured above). If the gluten is well-developed, the dough will stretch into a paper-thin film without breaking. If quickly breaks...you guessed it, keep kneading.
5. When You're Tired - You may laugh, but this is true! If you've been kneading for 10-12 minutes and your arms are tired, the dough is probably good. (If you get tired before that time is up, it's ok to rest for a few minutes and come back to it.) As long as your dough is close to passing the tests mentioned above, even if you're not 100% confident, you can consider yourself within range and call it a day.
For a more detailed look at kneading dough and how to tell when it's done, check out this video:
• How to Knead Bread Dough: The Video
Do you have other tips for knowing when your bread dough is kneaded?
(Images: Faith Durand)