If you're already giving insulin, you need to be home blood glucose testing before making any food changes, so you can monitor the impact. Switching to a low carb food can reduce the glucose 100 mg/dL and drop the insulin dose by as much as 2 units.
We recommend testing before each shot, so you can determine if it may be safe to give insulin. Testing at the nadir, the lowest glucose level after a shot, helps determine dose adjustments. You want this number to stay above 50 mg/dL on a human glucometer.
You need an inexpensive human glucometer, matching test strips, and 26 to 28 gauge lancets to test. Many of us use the WalMart ReliOn Confirm or Confirm Micro. If you avoid WalMart, use our SHOP link above and go to American Diabetes Wholesale. There, you can order the unbranded version of these meters as the Glucocard 01 or 01 Mini. All are made by Arkray and have interchangeable test strips.
We have some reference numbers for cats when using human glucometers. See my signature link Glucometer Notes for some details.