I don't think its actually possible to push your knees out too far. Your toes are probably at too much of an angle for your knees to track with them, and when you try to push them out, your knees tell you to stop doing that.
Y'know I always pictured a squat gnome to be like the most perfect squatter, with like a 600lb squat and the rest of the lifts < 400. And the gnome doesn't walk around, he squats around.