How do you know if a quail egg is fertilised?

There are some scientific and high-tech ways to tell, but these aren't affordable for the small-time producer. The best way is still the old stand-by: put then in the incubator and then candle them OR you could crack one and see if there is a "bullseye" spot on the yoke. That would at least tell you that some of your quail eggs are probably fertile.