Say, your might go to an auto parts store or Wal-Mart and buy an antifreeze tester. It looks like an eye dropper with 5 little balls in side and a little rubber hose on the end. How it works is you suck a sample of you coolant into the eye dropper thing. The littale balls will try to float. If say 3 balls float that mean you safe to about 5 degrees. If all 5 balls float, your safe to about 40 below. They work pretty well and are pretty reliable. If you add antifreeze and then try to test it it will give you an a phony reading. When you add antifreeze, you want to start the engine, let it warm up, and the let it run 10-15 minutes to circulate and mix the new antifreeze with the rest of the coolant. Let engine cool of so you can take off radiator cap and then test coolant. This will give you a pretty accurate test result.