I usually keep my leftover seeds from year to year, in the refrigerator. Although occasionally some seeds turn out to be useless, for the most part, most of them remain viable for years.
In 2010, I purchased some seed for green manure, but used very little. Not wishing to spend money for fresh seed if I did not have to, I decided to test the seed for viability. This is a very simple procedure: soak a paper towel in water, place 10 seeds on the paper towel, fold the towel several times, to trap the seeds in its moist environ, then wait.
The barley sprouted in about five days...
... with 90% viability. In other words, 9 out of 10 seeds sprouted.
So now I know I don't need to purchase more barley. I am also testing winter rye, which takes 10-14 days to germinate. Results shall be forthcoming.
It is sunny today, about 50 degrees, very springlike. Tomorrow is supposed to be similarly mild. Maybe it would be a good day to get some horse manure.