Have you gradually reduced the insulin to nearly nothing and had to skip shots? That is a common path to going OTJ. Basically, though, you just test, feed, snag mid-cycle tests to observe, and if you make it 14 days without going over 120-130 mg/dL, you're in good shape.
One clue is if you feed and then instead of a food spike in the +2 hours post-meal, the glucose goes down. That is a sign the pancreas is working.
The cat will always be diabetic, so never give high carb again. Monitor for hyperglycemia indicators - excessive urination, thirst, hunger, weight loss, and dehydration (tips in my signature link Secondary Monitoring Tools) andtest the glucose if observed. Some of those indicators show up with hyperthyroidism or renal disease, so they are good to routinely monitor.