When the pod is brittle and begins to split, is when the seed is ripe. Hurry and harvest it, before the wind scatters the seed in the garden and it is lost to you! Some will germinate there, but most of us want to sow it in containers, so we have some control over its growth.

Some seed will be 'fat' and often you get a lot of fine 'thin' seed 'cases' which is actually chaff and doesn't contain viable seed. It would appear all the seed on the left is viable, but to be sure, you can view it with a light source behind it. .

This reveals the embryo in viable seed. Chaff is empty. Here you can see some seed with the embryos visible as a 'stripe' in the centre