I could tell at day 4 if something was happening. You can try candling an egg before it goes in bator just to compare what you see in an egg that had already been in bator. you can see a difference in the two eggs. just don't throw them away right away, wait till you recheck them at day 7 or so. (if you candle before day 7) some people don't candle that early or very often or at all, I like to candle to see whats going on. lol Good luck, ~Julie~