Healthy nitro powder has a tolerable, even pleasant, solvent smell - reminiscent of acetone or lacquer. If powder has a stinging/chocking smell, like nitric acid or releases yellowish/brownish fumes, dispose of it AT ONCE. Flush it down the toilet or sprinkle it on the law as fertilizer.
DO NOT put questionable powder back in your stash.
Unstable nitro-compounds are a bitch especially in larger quantities. At a minimum, you can have a very serious fire. Worst case scenario is that double-based powders with high nitroglycerine content like Bullseye, WW231, HP38 stored adjacently can detonate under the 'right' conditions. Reaching those conditions in a storage situation is extremely unlikely but for the potential consequences I can tell you from (legally obtained) experience that a pound of WW231 is just as good as a pound of commercial dynamite for breaking up a beaver dam.
The good news is that decomposing powder releases fumes that the sensor in the middle of our face can easily detect. If a powder starts to smell 'funky', get rid of it.
WARNING: The decomposition fumes are
toxic. Do not stick your beak into the jug and inhale like a glue sniffer. Open the jug, fan the air above it towards your nose with your hand and if you get an unpleasant, chocking sensation, STOP.