From my limited experience: You place the eggs in 100F water and they should float. More importantly once the water settles; you should see distinct movement from the egg. You can't miss it. If there is no movement the then it is likely the embryo had quit. Use it if they have gone passed the normal incubation; especially on dark eggs.
YOu probably have a bad egg. They will sometimes look dark like there is growth. But you should be able to tell a couple of ways.....first of all, a non-growing egg does not hold heat.
Also at day 23, if anything is alive, you can tap on the egg while holding it to your ear. You will 9 times out of 10 hear the chick scratch back.
Lastly, what I do, when i doubt, is take a very sharp object and carefully poke a hole in the top of the egg. I clear away a bit of shell and look to see if the chick is alive or not. If it is, then take note if the veining has retracted and if the chick has pipped through the membrane.
Yes, you do risk shrink wrapping, but some warm water and a bit of time helping the chicks is better than a dead hatch.
I would be careful of the "smelly" ones. If you do have a rotten one, it will be horrible when it explodes. Check to see if any of the eggs are weeping or oozing. Throw them away immediately - outside. You do not want one of those to explode on you or in the house. it takes forever to get rid of the smell.
I guess I'll try floating them again and wait for movement. I never seem to see anything when I candle these green eggs. Chick that hatched yesterday is getting very lonely so I hope to get at least one more hatch. Otherwise it's off to the feed store and hope they have day olds.
Did the float test and nothing moved. I chipped of top of one egg and it had never developed. 2nd egg had a fully formed chick yolk partially absorbed. 3rd egg I put back in the hatcher cos I dont have the heart to open it yet.
will it work for a day 15 egg? i have one that does not look like it's growing anymore, the rest are all getting bigger dark blobs, but this one still looks the same as it did several days ago. would it be moving enough to tell anything?