To test old seeds, sprinkle as many as can be spared (at least 20) over a few layers of damp kitchen paper, keeping the seeds well apart, then roll up and place in a container (to prevent drying out) and keep in a warm place for about a week, then unroll and check to see if any have begun to sprout. If not, roll up, keep damp and warm and wait a further week, by then a few should have sprouted. If none, you might as well discard the lot.
It is all a matter of percentage. If out of 20 seeds, only 5 have sprouted, then generally no more than 25% of remaining seeds will grow. Having said that, it is recommended that 100 seeds are checked for viability to find the true percentage of growth, but often we don't get that many seeds in a packet in the first place.
Once we grow our own produce, it is very easy to save our own seeds so we don't really need to bother about age. On the other hand, a pack of lettuce seeds might contain 500, so if intending to grow each to maturity, not all will be used the first year of purchase. So just as well they can be kept for a long time.
Hope the above has been of some help.