When i check a clutch for fertility, i tend to have a better hatch rate. with this method, i hatch about 85%.
A Dr Pepper can with a hole in the bottom (about 3/4 inch in diameter) and the other end opened with a can opener).
i put a short two cell AA LED flashlight under the can (turned on) and set the egg on the hole on the other end of the can.
check it for the dark spot, inside on the yoke. LED flashlight is the choice because it doesnt produce any excess heat on the can
to transfer to the egg while it sets on the can, and it puts out enough light to luminate the egg so you can see the spot.
with quail eggs (especially spotted eggs like coturnix) you gotta look all the way around the egg to get a good view, so the
egg is on there a good bit of time. A candle, you could potentially harm the egg...