Pretty simple. I used this seed starting tray to hold little bundles of damp paper towel and in each I wrapped a bunch of seed from a few questionable packets. Put the lid on the tray and tucked it away somewhere and waited. No need to add water or anything. Just wait. I checked on the tray after a few days and it was obvious which seeds were still good and which were a waste of time. Then I forgot about the tray for a week and discovered some thriving sprouts. These were the biggest: beets, radish and arugula. No need to buy more of these seeds for this season.
In a proper test, I would have counted the number of seeds in each pack and the number that germinated. Divide germinated count by total to get the germination rate. Instead, I just eyed them up and if it looked like more than 60% or 70%, that was good enough for me. Of the 24 seed packs I tests, most tested adequately and a few were clearly duds. But there is still hope for some of the seed with poor germination rates. Now that I know which seed packets have a lower germination rate, I can put out lots to germinate and only transplant the few that sprout into proper seed trays to grow. For example, I had some tomato seeds that didn't germinate too well: something like 3 in 10. Knowing this, I won't be seeding them directly into individual seed tray cells. Even if I plant 3 seeds per cell I know I'll end up with lots of empty cells. Instead, I'll plant all of those seeds into a single tray and once they sprout, gently transplant them into the cells of the seed starting tray. Or maybe I'll do what I did for the germination test: wrap a bunch of the seeds (maybe the rest of the packet) in some wet paper towel and pick out the healthiest sprouts to transplant in the seed starting tray. I've done it both ways with similar results. The paper towel way is easier to set up but you have to keep an eye on them and get the sprouting seeds transplanted before they get too big.