I was just given a bunch of old Japanese chisels. I noticed that the hollow in the back of most the chisels has  receded form the edges quite a bit. I understand that this is not good because that means that the back has been flattened a few too many times, eating away at the hard, cutting steel of the chisel. 

My question is the following. Is there some (usable) hard steel left as long as there is a hollow left? Is there any way to determine when the chisels are not usable anymore?

Thanks in advance!