Hi Laura,
It is very easy! Any pod that stays flat like a snow pea never developed seeds. Pods that develop peas will get plump. Some pods will be totally full of seed, some many only have one or two good seeds so they will be half plump.
Let the pods dry on the plant. Collect them all and then open them up. Good seed will be hard dried peas just like the ones you planted. Little tiny seed that never developed could be in some, but you will definitely be able to tell the difference.
Remy