There are various simple tests for viability. One is to dampen a plain white paper towel and fold it in half, place a few seeds one one half of the towel and fold it in half again over the seeds, enclose it in a ziplock sandwich bag and place it in an environment appropriate to the seed's germination requirements (light, dark, warm, cool, etc.). After a week or so, check to see if any sprouts have appeared.
Some seeds, such as peas, can be tested for viability by placing them in a bowl of water. Those that float are sterile (contain no embryo and are therefore lighter); those that sink are likely to be viable.
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