We see this in very old sheep which are starting to lose their molars. They get some nutrition from chewing the cud before they drop it, but eventually they waste away. You can feel their back teeth from the outside so you might be able to feel any gaps and you might also pick up any abscesses or similar problems, before you have to look into the back of their mouths which they resist like mad
If you can watch your sheep while they are cudding you may well pick out the problem one as she will have the wodge of cud hanging out of the side of her mouth some of the time.