Smell will tell you what you want to know, if its off you will know without doubt, if you have to think about it its all right. It really vrots and gets a interesting mold growing on it. I have had one tin go off in my life, it was a almost empty tin of MP 200 stored for 15 years.
I've loaded with Musgrave S200 branded powder in the square tins from the early 80's that was within shot to shot velocity variation of the last batches of MS200 that I had on hand. I have no fear whatever of old powder or primers as long as they show no signs of shitty storage.