What I have done in the past, when in doubt about the viability of seeds, is to put 10 or 20 of them between a few layers of damp paper towels. Keep the towels moist, but not soaked, and wait for however long it usually takes for seeds of that species to sprout. It isn't perfect, but it will give you an idea of the kind of germination rate you can expect.
If you put 10 seeds out, and only 2 sprout, then you'll probably get a germination rate of 20%. IOW, 20 out of every 100 seeds are likely to sprout when you plant them. OTOH, if 9 out of the 10 seeds sprout, you can expect a germination rate of about 90%.
Marigolds usually sprout within a week to 10 days after being planted, I think.
What kind of marigolds did you collect the seeds from? I have read that the seeds from the African-type marigolds ... those are the taller varieties with the big flowers ... are usually sterile. The marigolds with smaller flowers are called French marigolds, and they should be fine. If you started with a hybrid variety, however, the plants that grow from the seeds you saved very likely won't look exactly like the parent plants.