I just scored the 3 pak of St. Louis ribs at CostCo. Hoping for a break in the weather this weekend to sneak in a cook on the KK.
Dennis is an advocate for the "bend test" to check for doneness on ribs. Pick up the slab with a pair of tongs and bounce them slightly. If they are ready, the slab will bow until the meat starts to crack on the surface, as shown in the picture. A small crack means you need a little more time. It should be close to breaking when you lift the slab. This is "competition done" (i.e., with a bit of bite to them, not "falling off the bone," which is actually overdone. Or worse, a sign that they were boiled/steamed first!)