The dough should feel smooth, elastic, and soft. I like to test my dough using "the windowpane test". Basically, you need to tear a small ball and stretch it into a square using your fingers. If the dough becomes thin enough so you can see through it, you've kneaded it long enough. You can stand next to a window to see if light passes through it. If the dough breaks, you'll need to place the ball back with the rest of the dough and keep kneading.