I candle my Coturnix quail eggs after about 7 days to see if there is any veining, but once they start fully developing, its almost impossible to see the chick like you would be able to with white eggs. When i transfer the eggs into the hatcher i check again and just go off of color, if it lights up like a clear green light bulb, its infertile, if its solid and has a good air sac, in the hatcher it goes!! I use an LED head lamp and it seems to work just fine.
I candle the coturnix, they are pretty easy early on. I go by the 'fullness' the closer they get.
Buttons, I can candle them, but basically all I can see is the air sac, and if the rest of the egg is 'dark' or 'light'. If there is a late quitter, I can't tell. But there are some button eggs that I can't see into, those just have to wing it.