I also just check on air cells. By the time day 17/18 rolls around I have a pretty good guess which ones are clear, but I still leave them in the 'bator for fear that I am just not seeing what I think I'm seeing through those dark shells!
Thanks for the help. I don't have a 'surefire'. I was going to use a flashlight set-up. I've got a super powerful flashlight and I was going to put a piece of black paper over it and only let the light pass thru a small opening. Isn't that the idea?