I'm still using up powder I bought from a guy who was getting out of reloading 15 years ago. Some of that powder is over 40 years old.
Now I was reasonably confident that the guy knew what he was doing, so the risks were small (he reloaded for revolvers and shotguns). So that played a part.
He used the Hercules Bullseye, Unique, and the dot powders. So I examined the opened dot powders, and they were consistent and didn't appear mixed. There was also an opened can of bullseye and one of unique. No dots mixed in them.
One thing I did do (for the opened cans) was look through the reloading manuals was find common uses for the opened powders, and figure out what min and max charges for each one. Dealing with Bullseye was easy - just load for it and see what it did. I loaded up a few rounds with unique as if it were bullseye (they overlap in 9mm). The unique became a 45 colt powder, so I laddered those the first few rounds up to ensure everything was as it should be. The risks were low, and I had enough experience by then that I knew I'd be able to determine what I was working with. I knew squibs were a possibility, and made sure each round went downrange. I also started off on the low side with the dot powders, just to make sure they were consistent with expectations. Eventually it was clear that the powders were in their correct containers.
BTW, a couple of those old powders seem slightly milder than their newer versions. I assume they've lost a little energy over time.
Anyway, I don't recommend experimenting with powder in every situation, but if someone is willing to put in the time and effort, and doesn't hesitate to take a loss on anything questionable, or where there too many risks, it is possible to figure out. I would not do that with rifle powder though - the risks are way too high.