I use a LED flashlight from the hardware store. I go in a completely dark room, (bathroom at night) and hold my hand all around the light so no light escapes, and start at the bottom of the egg, and slowly go up the side of the egg, at around ten days. If I don't see an air cell clearly defined at the top of the egg by then, I toss it, unless it is just too dark to tell. Also, if it just lights up like a little japanese lantern, it's a dud.