Most of the once-fired brass I use is .223, but I have bought .40 brass before. The telltale sign to watch for is the rectangular firing pin imprint of the Glock, I don't know of any other gun that does that. If you are using once-fired brass, check the fired primers, and pitch the ones that don't have a round dent.
BTW, my Witness 40 leaves a rounded dimple, plus a wipe mark to one side, almost teardrop shaped. It means the shell is being ejected before the firing pin has fully retracted, but it's never affected function or reliability, so I don't worry about it. I'm closing in on 32,000 rounds thru this gun, and it has never jammed, with any ammo, with any nose profile. And brother, I've tried them all. I spent a year trying to load rounds to choke it, all nose profiles, loaded long and short, and it gobbled them all up without a single burp. If have lots of guns I love, but when I need absolute reliability, the Witness gets grabbed first!
Papajohn