Candling dark eggs at this point is actually pretty easy for what you are looking for. The eggs should be full of chick so the main thing you are looking for is to see if they have internally pipped. Look in the air cell for the beak. I had a late hatch on my first time around. Three of them started hatching on day 22. After they finished, I went to candle the remaining eggs and two of the eggs "peeped" at me when I picked them up. I didn't even bother candling them since I knew I had live chicks in those. Listen real close to the incubator and see if you can hear them peeping. If they have internally pipped, you might be able to hear them. I couldn't hear mine with the cover on the bator beacause of the fan. Good luck and it is way too early to give up yet.