Break open the egg before it hatches on it's own. Alot of times when it gets to that point the baby inside is already dead but either way too weak to hatch.
Our general rule of thumb is Mold or day 70 for 82 degrees. Whichever comes first, but, a little mold isn't a bad thing most of the time. You can always smell it as well. If it smells like death then it normally is dead.