Other ones to avoid:
- If it's badly scratched from being stepped on on rocky gravel
- If it's badly corroded
- Some brands come with primer pockets that are just a hair too small for normal primers. You can ream them out or use special primers, but it's a PITA. S&B 9mm comes to mind. Winchester NT ammo uses odd sizes - small pistol primers for .45. All that's a personal choice.
- similarly, MILSPEC ammo will generally have the primer crimped in place, which will make reloading a new primer difficult. Once you swage or grind or drill out that crimp, they last forever.
- anything with Berdan priming or non-brass cases (which do turn up from time to time)
- any piece of brass somebody much larger than yourself is reaching for at the same time.
Oh, and physically look into each case you glean. I broke a depriming pin on a rock wedged inside a 9mm early on. Learned from that and my roommate of the time learned some new words.
Woops! Late edit! If you're shooting range ammo, check the rules with them first. Many ranges sell cheaper ammo so they can resell the brass. Some will get very pi**y about you helping yourself. Check first.