Another clue is if it was fired and reloaded. New brass reloaded could be tricky but most people are going to sit the bullet out farther than the factory. Factory ammo has to be able to chamber in any rifle so the bullet has to be seated farther in the case. Reloaded after fired is a dead giveaway to anyone who is experienced to reloading. The shoulder area was fire formed to what ever chamber than just sized enough to be able to go back in. It will look very neat and uniform. Again factory ammo has to be able to chamber in any rifle so there is plenty of slop in shoulder and neck area.
-Full length sizing of brass.
-Brand new, or tumbled brass, doesn't show signs of being fired.
-Utilizing crimp grooves or loading to a COAL for reliability, rather than greatest accuracy. For example, I actually seat the Nosler Varmageddon bullets I use in my .223 much deeper than I care for, but I need to for them to cycle out of the magazine reliably. Lever action and handgun ammo aren't tolerant of huge variations from factory either.
Got to come back at you on this one. Take any caliber open up a box of factory ammo pull 2 out. Fire one than reload it, go complete process fl, tumble , give it 100%, etc. Reload it and compare the 2. The shoulder neck area will look different. Or maybe I should say in our eyes of experienced reloaders can tell but maybe average hunter would not see though.
Another thing I might add if you by chance watch the Moonshiners show on Discovery channel on Tuesday eve? Few weeks ago the sheriff dep. trying to find out who shot his trail camera picked up a few spent cases than measured oal of case and made assumption it was a reload just by that. Guess you have to watched the show if you did we can debate that one. I really didn't buy in just wanted to hear someone elses opinion.
You are assuming the OP is referring to a case with a shoulder vs a straight walled case.
You are also assuming that the OP has a factory round to compare it to.
I will grant you that there are some dead giveaways to reloaded cases. I have enough experience to make an educated guess as to whether a round is a reload or factory, but not enough to stake someone else's safety on it. If the OP is asking how to tell the difference, then he is likely not an experienced reloader.
I always wonder what will happen with some of my guns and ammo if I am to die young. My custom rifles are not marked with their chamberings, and my wife has no idea what most of them are. On top of that, I have three cartridges with incorrect headstamps. I own a .20 Practical and a .223, both use .223 headstamp brass. My 25-06 barrel is unmarked, and the brass is necked down .270 brass (from when I had a .270). The 243 Ackley Improved is the factory headstamp, but at least that won't fit in anything else. I've considered taking pictures of the gun, and it's corresponding ammo, and making a book for her to reference in the event of my untimely death. Turns out I'm really good a procrastination though.
Yes I was assuming someone knowing what they are looking at not average Joe. I have the same issue my self. Most of my rifles are wildcats but i will say with the net these days our wifes could have a better chance. Years ago it would of been in a small circle of someone actually knowing what they are. My one wildcat on the barrel is stamped 25 Bobcat. Its some thing few years ago I had reamer and dies made actually 270 WSM necked to 25. Its worth a lot investment wise blueprinted 700, broughton barrel, McMillian stock, but who would actually pay for it. Guess it is something for us to start thinking about.