Should I stop turning my incubating eggs?

I received 2 duck eggs from a friend that I've been using in my incubator. The problem is I know that I am suppose to stop turning the eggs 2 days before they hatch, but I'm not sure when the hatch date is. I've been candling them regularly, and they've been very lively. It has been about 3 weeks and you can now see them struggling in the air sack (which takes up nearly half the egg now). How can I tell if they are trying to peck their way out and that I should stop turning them. What if I already screwed it up....if I candle and see the beak should l lean the egg onto it (i.e. turn the egg with beak facing down) so gravity will help them hatch? Thanks!!