How do we differentiate which lettuce seeds are good and capable of germination, from the rest?. I planted lettuce seeds, and only 'Radical' came up, after the rest could not germinate.
The classic viability test for is to place a counted number (10, 20, 50, 100) of seeds separate from each other onto a damp paper towel or blotting paper, supply them with optimal germination conditions as far as temperature and humidity are concerned and wait the standard time for germination.
After that, count the number of seeds that show signs of germination and determine the percentage of viable seeds in your batch.
Depending on your results, you'll know whether you have a fully viable batch of seeds (like 90% germination), one with a reduced germination rate (sow more than usual) or very small germination rate (don't bother...).
More on germination testing here, for example.