You can get a pretty good idea if a case has been reloaded just by taking a real close look.
Find out what color of primers a company uses in their factory loads. (if you find a remington case with a silver primer...probably a reload)
Look for the primer sealant used by manufacturers, if its still there, most likely not a reload.
Check the length of the case, is it what you would expect to find for a once fired case. Look at the brass to see if there are any signs that it has been trimmed to length. Untrimmed brass almost has dimples on the casemouth, looking at it from the end instead of smooth (trimmed).
Look at the case head and see if there are multiple extractor groove marks, indicating reloads.
Are the primer pockets sloppy?
If its militarty brass, has the primer crimp been removed? If so probably a reload.
Some people only wipe the sizing lube off instead of tumbling. The brass still will have almost an oily feeling to it, another indication of reloads.
It is much easier to tell if a rifle case has been reloaded than a pistol, but at the same time when a pistol case has reached its useful life the neck will generally crack making it very easy to tell if it is safe to load again.
Since I don't keep track of the number of times that I reload my own brass, I use the same criteria to determine if a piece of picked up brass is suitable to be reloaded. To be honest using the "clues" that a piece of brass will give you , I have had zero problems. I mostly shoot bolt guns and revolvers, so feeding issues could come into play for those with auto loaders.
