Corned beef should be tested for doneness the same way you would test a roast. When the meat is fork tender, it’s done. You can stick a fork into the corned beef, gently twist, and pull the fork toward you. If a piece easily comes away, it’s done! If the meat is still one homogenous piece that can’t be sampled with a fork, then the connective tissues haven’t broken down yet. Broken connective tissues = tender meat you won’t spend all year chewing.