I can't candle tonight... I can't candle tonight... I can't candle tonight. (Most of my eggs are very dark Marans.) How do you guys that hatch all the time check them? I'm planning on just using the sniff test for now.
I candle them, but I also sniff test. I don't do anything with my sniffer unless it detects an undesirable aroma when I open the bator...I don't smell each egg individually unless I have a reason to. I candle on day 1 (to check air cells) day 7 (to check fertility), 10 (to check viability) and 18 (for late quitters). I used to candle every day with my first hatches, but have since learned that the strong LED lights may have an effect on the chicks after they hatch (temperament wise)...so I don't do it anymore.
I also check them by weight, and actually...thus far it's been 100% accurate (for me anyway). Picking up the eggs I can tell which ones have chicks developing in them and which ones are either developmentally delayed or perished, or infertile (due to weight of the egg in-hand). But - I still candle to verify, because I'm human and humans screw up. ;-)