Get yourself a roll or paper towel, and some sandwich bags. Cut a section of paper towel (fairly small, you don't need an entire sheet for each seed variety) and get it wet. Lay out 10 to 20 seeds of one variety in a line (don't go crazy getting them perfect). Fold over the paper towel, so the seeds are covered and in contact with the moist surface. Put the seed/paper package in a plastic bag and label with the variety, date, and number of seeds. Put the bag in a warm spot, but out of direct sunlight (you don't want to cook the seeds).
|Tomato seeds on moistened paper towel|
|Bag it and tag it!|
Start checking the seeds after 3 or so days. Germination may take a couple days to a couple weeks, so be patient. Most of your seeds will germinate within a day or two of each other. Once this flush of germination is over, count the total number of seeds that germinated. Divide this by the total number of seeds to get your germination percentage. For example, if 17 out of 20 seeds germinated, you germination percentage is 85%.
I would consider a germination percentage of 80% or higher to be sufficient (90% or higher is even better). However, acceptable germ % can be lower for plants that are known for low germination (like certain gourds, parsnips, and some flowers).