Grass seed is typically good for a year after it was tested in a laboratory. As long as the seed was stored in a cool, dry spot, the germination rate on the package should be valid. If the seed is in a hot or moist location, the viability is compromised.
The viability of seed decreases for each year it's stored. The germination rate may drop by as much as 10 to 25 percent per year. That means the amount of seed that actually results in grass is reduced, so it's a good idea to spread more over the year to compensate.
You can test seed by taking a handful and blowing across it lightly. Seeds that blow away have died. If you notice a lot of them blowing away, the seed batch is no longer good and you should buy new grass seed.