When it smells like somebody farted in the can, its bad. Or if it is obviously broken down and gotten wet and clumpy then its bad. Other than those things I haven't seen or heard of powder really going bad. There are guys on here and some others that I have met that are still using powder that they bought by the keg full in the 50's and 60's and they are still getting the same types of groups they got when they bought it.