perrazi is on the right track, SMELL !
If the powders have a "solvent" (alcohol, either, acetone, etc) smell, they have not begun to deteriorate. It may smell to some people a little like black pepper, too - that's OK.
If you get an ACIDIC or SOUR smell, it is going. Those smells will be strong, right off. You WILL know.
Perrazi says an ammonia smell on firing - that is one of the very beginning things, but that can also be confusing, depending on the bulletlube (cast bullets) used. Some lubes produce truly wierd smells on firning, even with new powder.
Flash