There should be a date on the bag of seed - showing when it was packaged. Maybe start taking a sniff when you open a bag and see if you can detect an off or odd odor?
At the feed mill, they bag their own seeds and there is no date on their tags. In fact, they do not even have any tag on the bags at all so you better remember what you bought before you put it into storage. Also, the bagging date might not be as important as the harvest date of the seed. I think the bad bag of barley seed I got was some old stock in a farmer's silo and he just wanted to clear it out. At any rate, you get what comes out of their main chute and they only guarantee that the seed is fit for dry feed - not guaranteed to have a high germination rate for fodder.
I can get a 100# bag of barley seed for about $8.00. If the seed is good for fodder, that is wonderful. If the seed in that batch is not good for fodder, I just mix it into my chicken scratch and it's still a good deal. Either way, the chickens will eat it. I just like to grow fodder in the winter as that is the only greens my chickens will see until the grass starts growing the next spring. It's a nice treat for the girls instead of just eating their commercial layer feed.