Never remove the radiator cap when the engine is hot. Always wait until the engine and radiator cool down.
Check the coolant level in the reservoir tank when the engine is cold. If the coolant level is below the "MIN", add coolant up to the "MAX" level.
If the reservoir tank is empty, check the coolant level in the radiator when the engine is cold. If coolant level is not at the top of the radiator, fill with undiluted coolant up to the filler opening and also add it to the reservoir tank up to the "MAX" level.
NOTE: Some of the older vehicles may not have a coolant reservoir tank. In this case, just remove the radiator cap and check the coolant is up to the filler opening.
Easy as that! The coolant level has been checked.