I have some Watermelon seeds from 2001. I planted one and I have a little watermelon plant in the window! I have Cantaloupe and Pumpkins from the same year and have had no luck. Im going to pull out my seeds and try the paper towel in the ziplock bag idea though. good Luck!
I keep my seeds in the fridge in an air tight container. They last for years and years. Since I do Square foot gardening I only use a few seeds at a time, when I plant.
I suppose if one wanted to test if the seed is still viable, would be to attempt to sprout a few of the seeds in question.