First, brass does have a lifetime, meaning it can only be loaded and fired so many times before it has been stretched past normal dimensions, and is dangerous to load. Books I've read suggest that 6-8 loads will be about the max you can expect from a handgun case. Chances are if its found at the range, you can assume safely that its only been fired once. Most reloaders collect their brass after. This is good. Now you know where you're starting from. Keep track of how many firings they go through. I use ziplock bags to keep different batches separated and labeled. Once you've sorted out the obvious no-no's, like dents, cracks, holes, jagged case mouths, severe corrosion, etc., run the rest through a case tumbler to remove the dirt, tarnish, and powder residue. Then give them a good once over. Be ultra critical. Look for any signs of overpressure, case head separation, or overthinning of the case walls. Do this before you deprime and resize them! Resizing removes some of the evidence of deteriorating case wall integrity, and looking at the old primer will help you determine if the previous load was over pressure. Reloading manuals will have pictures of all of these, and more examples abound online so you can know what to look for. Toss any that look questionable. Also, be sure to take a peek down into each one to make sure you only see one flash hole. If you see two, toss it: that's a berdan primed case, and you'll bust your deprime pin. Nickel plated cases are nice because the nickel will wear off right at the pressure band, which is just above the case head. When this happens, you know you're nearing the end of the case life. I normally see the nickel rub off right around the third or fourth load. I give them one more run after that, and toss em.
Cardinal rule: be super critical about what you choose to put back into your gun. No amount of savings in brass/ammo are worth a catastrophic failure in your gun, and possible injury to yourself or others.
Happy holidays from a fellow brass hound!