I'd say when they look close to eating size, taste them! The ones that taste good should be the snow peas, and the others should be tough and not so delicious.

Good luck! I did the same thing with onions this year -- 1 month storage and 12 month storage that looked the same and I'd forgotten which was which. A slice test gave the answer (different sized rings)

Many varieties of snow pea have colored blossoms. Unless you planted an exotic variety, shelling peas have white blossoms. The difference in the pods is usually apparent in a couple of days and before they get full size.

Very quickly.

And if you squeeze the shelling pods, you should see where they pop open, unlike the snow pea pods.

As pea pods get bigger, they tend to stay flat and wide before seeds start to develop (at which point they are usually too mature). I think the seeds in shelling peas expand as the pod gets larger and the pod starts getting round right from the beginning. I grow peapods and sugar snaps. Once the pods start showing, I think you'll be able to tell the difference. They aren't going to stay on their own side of the fence.