Leftover grass seed can be sowed after the year of purchase if you stored it in optimal conditions. You can test grass seed at home to determine if it will still grow.
According to scientists at North Carolina University's Department of Agriculture and Life Sciences, grass seed will last approximately two years when stored at room temperature. Seeds stored in cool, dry conditions stay viable the longest, sometimes even beyond two years. Texas A & M University experts recommend storing grass seed in an airtight container with a packet of silica gel to protect it from moisture.
Specialists at North Carolina University's Department of Agriculture and Life Sciences recommend against storing grass seed in garages and storage sheds, where high temperatures can quickly destroy the seeds' viability. Dramatically fluctuating temperatures can also cause seeds to deteriorate.
For a simple sprouting test, Yardcare.com suggests planting a few seeds in a small pot filled with soil. Cover the pot with plastic and see what grows after a week or so. Or follow the recommendation of North Carolina University's Department of Agriculture and Life Sciences and place 100 seeds, evenly spaced, between three layers of moistened paper towels. Roll up the towels and place them in a plastic bag. Place the roll upright in a can near a window but away from direct sunlight. In 10 to 14 days, count the sprouted seeds to determine the germination percentage. Adjust how many seeds you plant accordingly, or throw away the seeds if only a few sprout.