I was using a box-with-hole and lamp method and realized when I chucked 3 viable eggs that home-made wasn't so swift. I couldn't really see much of anything, just whether I had a clear egg or not.
THEN.... I sent away for an got a Cool-Lite tester. OMG!!! It's like watching Egg TV! I could see all sorts of veins and detached air cells and blobs and even a future double-yolker (Hey! There's two embryos in this one!). Best of all, you don't have to move the eggs if you can get your room dark. You can test in the bator! Best $$ I ever spent.... well, maybe it shares that distinction with the automatic egg turner.....