For the first 10 days I usually can’t tell. After day 10 you can usually see a contrast with the air cell in the eggs that are developing and then you candle from the bottom you can gauge the size of clear space vs. darkness (aka where the chick is). It’s not easy and I’m often unsure.
It's day 13 and I candled yesterday. One looks pretty clear but I hate tossing if I'm not positive. I'll just wait until they smell and recheck before lockdown.I don't candle until day 11. At that point I can see that something's going on. Then I candle again right before lockdown, at which point they just look full of darkness/chick.