The maturity of the seed affects a couple of factors , the colour is an indicator , as is the outer shell. Immature seeds are often light in colour and a little small and fragile .
The shell should be firm and the colour should be mid brown to black.the seeds should also be of a reasonable size.
This is a generalisation but in 80% of cases I would be looking for these factors when choosing seeds.

Obviously the more mature a seed is the more likely you are to grow a healthy plant.