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In the cases I've helped you can tell because it has been hours or days since the initial pip/hole was made and not any progress since. The chick is still in same place and usually it's beak/face is smooshed out of hole. By enlarging the hole slightly, if necessary, you can tell if there is any room left in egg for chick to move. You can usually see the open air cell end if there is room. However, when the chick is really big you can see it just kind of pushing against shell in every direction with no empty space in egg. The eggs I've helped zip I thought would hatch right in my hand or burst open because every time I'd pull a tiny piece of shell off, the egg would expand. I could actually feel it and see it expanding in my hand as I made a zip arond it. I could watch the membrane expand and stretch.
As I mentioned I've had to hatch a lot of my Barred Rocks but never had to help my little Ameraucanas which are small and when they pip they quickly zip all the way around and you can easily see they have lots of room in the shell.
I run my bator all the time and always have a mix of eggs: chicken, duck, peacock all hatching daily. In one hatch I had only BRs and AMs and had to help all BRs and none of the AMs so I don't think it's a humidity or temp issue I just think some breeds or chicks are bigger than others and can't zip the shell. Ever since that first learing experience I've never lost a chick but I only intervene when I know I must. I try and give it lots of time on it's own especially after I make sure it's beak is out and it can breath. I take each chick out of bator immediately after hatch and put in brooder under heat lamp with wet crumbles right next to it on floor and it will start to eat before it can even lift it's head. By getting them out of bator they aren't knocking the other eggs around which might be on anywhere from day one to hatch day.