Actually, to check your coolant, you need a coolant tester, which looks like a cross between an eyedropper and a turkey baster, with little plastic balls inside it. You draw some of the coolant into the tester by squeezing the bulb, then look at the little balls. They float at different specific gravities, and by comparing the balls to the tube markings, it will let you know if you need to add water or coolant to get the proper mixture. You should replace your coolant every two years, and check your coolant every three months for the proper mixture, particularly in cold climates.