Since JP posted this
link, I've been using the "toothpick test" with consistently satisfying results.
"The toothpick test. Jack Waiboer, a champion cook from South Carolina likes the toothpick method. "You poke a toothpick into the meat between the bones. When it slides in with little or no resistance you have a done rack of ribs. It's best to test various parts of the rack to be sure you have them done to your liking. With a little practice you can better judge the doneness of your ribs than the bend test. I hesitate to use the word accurate in a barbecue conversation. After a few beverages accuracy seems to depend on the quality of the bourbon."