A pod that has reached it's full size has a chance of having mature seeds, but often the germination rate is lower. If you want to try them I would set out more starters than plants you hope to end up with, and if you're anxious to get results at a particular time you should just buy some seeds since that's more reliable than guessing whether the pods were mature enough.
... but to answer the larger question, seeds become viable before the fruit is ripe, but becoming ripe is a sign the fruit had matured as much as it will. You might leave one of the pods sitting out at room temperature and see if it starts to turn colors, if it starts to turn before it dries our or rots (or within about 2 weeks), odds are better that you have some viable seed though it's no proof, just because one pod is however near maturity as it happens to be, that doesn't tell you that the other pods are at the same stage of maturity though how near they are to what seems to be full sized is a good hint until the weather starts getting cold with shorter days and pods may not get to be as large as they used to.