When looking at used firearms, is a cracked stock a sign that the gun has been fired a lot?
Depends a lot on the rifle. A lot of the old Savage 99s would develop a crack in the stock near the tang.
I was going to make the same comment. I've seen that crack/chip on two 99's. One with plenty of use from the 1970's, one from earlier that was very well cared for. From comments it seems very common.
Most of the time wood stocks crack it's because it was dropped on a hard surface.