A germination test is a simple gardening technique that involves nothing more than the seeds, some absorbent paper towels, a spray bottle, water, a zip lock bag and a warm spot.
Take about 20 seeds and place them in a moist paper towel (not wet) and fold the paper towel so all that seeds are contained within the moist paper towel. Separate the seeds so they are not all bunched together, because you will want to count them in about 5 days. Then place the paper towel in a quart ziplock bag and seal the bag. Place the bag near a window where it can get some light, but not direct sunlight, where it will maintain room temperature of 70 degrees more or less. After 5 to 7 days you will gently open the paper towel to observe how many seeds have germinated and how many have not.
This experiment to find out was suggested by Craig Dremann, who also advised me to add fertilizer to break the mustard seed dormancy (but see below; this was not needed for the seeds germinated in a moist paper towel).