If oil is number one in the fluid pecking order, water, or coolant, ranks a close second because of its role as a heat regulator. Unless you have an older Volkswagen Beetle (or a not-so-old Porsche 911), you will want to check your coolant level as often as you check the oil. This should be done with the engine off and when the vehicle is cold, and must be done when the vehicle is cold if you are opening the actual radiator. Most late-model cars have an overflow tank (light-colored plastic with a rubber tube going into its cap) that can be checked to determine coolant level with no chance of scalding, but even these engines should be cold for an accurate reading. Once again, there will be marks on the overflow tank, often labeled "MIN" and "MAX," that will make identifying your coolant level a no-brainer. If you have an older vehicle with no overflow tank, open the radiator when the engine is completely cold and look inside to check for coolant. It should be visible just below the radiator opening.