Before we carry on lets get the legal mumbo jumbo out of the way.
As a disclaimer I do not know everything about reloading, and do not take what I say as golden. I am sharing my knowledge of reloading. I cannot control how or what you or anyone else reloads, and cannot be held liable for any injuries. damage, or death caused by you reloading. I always tell people too get at least one good reloading manual, study it, and study it good.
Now I am sure you have accumulated some shotgun hulls for reloading. Now that you have picked out your load, bought supplies (powder, shot, wads, primers), and you have a press you're ready to load. Right? You are WRONG!
You need to go through all your hulls, and cull the ones that cannot be safely reloaded or will drastically effect your patterns. Now first if there is any moisture in the hull throw it away. Glue is used in making a lot of hulls. Some even have paper basewads. When that glue gets wet it weakens as does paper. In turn when firing if the glue or paper has weakened enough the hull can separate in your firearm, possibly lodging in the barrel, and if another round was fired it could cause a catastrophic, and potential deadly firearm failure. It is not worth the risk. You should cull any hulls that you think might have become wet or immersed in water. There are plenty of times that you will see water marks on them or they're tarnished which makes it easier.
Another key one to look for is splits where the shell crimps. This is common in rounds that have been reloaded multiple times, and have reached the end of their useful life. Sometimes it will be easy to tell, and others you won't be able to tell until you are crimping the round. Even if it is not easily noticeable it can severely effect the pattern of your gun. Which if you are loading rounds for hunting or competition this can mean a wounded animal rather then a clean fatal shot or cost you a competition especially since one off shot can cost you a match.
The easiest way to check for cracks or splits is too hold the hull at an angle to a light, and slowly turn it in your hand.If you see ant light coming through the hull you need to toss it.
Another key one to look for is charring or a burnt hull.With a cooked hull (that is just a term I use it is not a technical term)all kinds of things can go wrong, very wrong. Which these hulls can lead to a catastrophic that can possible cause damage or destroy a firearm or severely injure or even be fatal to the shooter or a bystander.
Cooked hulls have black burn marks. Now you will be able to tell between a dirty hull or a cooked hull by rubbing your thumb on the black area. If it is not rough or brittle feeling or the discoloration doesn't carry over to the inside it can very likely be soot, and safe to reload. A couple other key signs are bubbling of the plastic or the plastic being brittle. Anyone signs of that, and you need to pitch it.
A key thing to remember is "When in doubt throw it out!" which is an important rule to remember in all aspects of reloading from hulls, and wads to brass, primers, and powder.