Danny, there aren't many visible signs of pending case failure. Frequently old tired brass will start showing tiny splits in the neck. These aren't catastrophic but signs the brass is finished and should be tossed. Any loosness in the primer pocket is a very bad sign and should be tossed. One that scares me is when I spot a case that seems to be forming some sort of "ring" or "crease" in the brass just above the head of the case. The last thing you want is a head separation on your brass when firing.

Here's a good rule: "When in doubt...throw it out."

Some reloaders also straighten out a paper clip or get a length of wire and bend a little "L" shaped end on it. They run this down inside the case and feel their way up from the base of the case across this head separation area. If they feel much of an indentation there in that area...watch out. In fact you can shine a bright light down the case and see this area.