How do I know when my crab apples are ripe?

From my experience, they always taste sour. Maybe I'm just not waiting long enough.

Used only to make jelly (in my experience) and hence sugared to taste. I can't imagine actually eating one. I would guess that when there is a slight red flush on fruit, they are ready.

Crab Apples turn a wonderful rich red and are fragrant when ready to pick. Also, you will notice that some fall off the tree with a shake of a branch. They are bitter though. As mentioned above, good as jellied.