I get the best results at day 3 to 4, no later than 5, to see if they were fertile and growing. You see a little bean in the center with a roadwork of veins coming off of it.
By day 7, it is mostly dark blob in 1/3 of the shell. By day 12, you've got nearly 2/3 dark shell blob with air space and large veins. After 14 or 15, it is hard to see much other than dark blob. By then if you hold very still, you can sometimes see and feel movement. I find it hard to tell the late quitters at later than day 7 to 10. It is easy to tell the early quitters and they stop developing while the other, growing chicks, consume more and more of the shell.
I am using a simple LED flashlight from the Dollar Store with egg cupped in my hand. Those who make more effort and expense get better results. I do this quickly and simply on the fly to eliminate the duds so that I have less risk of bad eggs exploding in warm weather or a persistent hen brooding on duds and ignoring new babes.
I hope your farm is doing well, you've always got something going on
LofMc