Tris
One thing I have tried is a quick germination test in small trays, putting five seed packets under test. I did this with all my lettuce seeds after direct sowing this year in modules was poorish. Last years and this years Canasta from Seeds of Italy; last years Little Gem from SOI and this years from Johnson; two year old Grenoble Red from SOI.
Attached photo at day 6: Johnsons Little Gem the best; both Canastas coming through OK;SOI Little Gem just emerging; Grenoble Red look dead.
I will prick these out after several early season struggles.
If you tried 20 onion seeds in a row sown e.g. January 19th, you would know which batches were OK before you needed to sow for real in mid February. That would probably not be more than 10% of seeds in a packet.
Lettuce is easy given the size of seed packets, as is turnip and radish.
I am tempted to do it with spinach and cabbage, as I usually have far more seeds than necessary. And pricking them out would not be a problem.