If you shine a small flashlight through the air sac you can often at least see if there are veins or if they've broken into the air sac. On very dark shelled button eggs this still will not work but I have been able to tell at least half when candling before I stop turning.
If you do decide to open one take something with a good point and lightly tap the very tip of the large end until it chips. Be careful not to go too deep or you could hit a chick. Make the hole big enough for tweezers and very carefully break tiny pieces off until you can see something. Don't break too large of pieces or the egg could split down to where the membrane attaches and break a blood vessel. Then you've most likely just killed anything alive in there. If at any point you hear peeping stop and put it back. If you get to the point you can see the membrane and there is a live chick that hasn't broken the membrane I would cover with a damp paper towel, put it back, and wait another 12-24hrs. I highly do not suggest trying it though. I tend to get impatient so I have done this several times with late eggs or the time an entire hatch didn't start until day 20. You will most likely kill at least one live chick the first few times you try it. Unless they are really late like end of day 19 to day 20 I would not even think about it.