I recently obtained a nice cache of powder from a friend. In the metal cans. Cans were rusty on top and on the inside. Rusty dust cloud when I poured the powder into the hopper.
Took it outside on a breezy day and dumped the powder back-and-forth between two containers, letting the breeze carry the rust particles away. The dust was from the rusty cans, not from bad powder. Powder passed the smell test, and it looked like, well, gunpowder.
I picked a gun, that if it got ruined, I would be the least upset. Strapped it to the bench, tied up 30' of string, loaded a round and hoped for the best. 5 test rds. per can. 4 cans. Bottom of the charge range to the top.
The choice I had was maybe blow up a $300 gun, or toss $500 of powder.
I now have enough powder to last for a long time.
