Q: Head gasket failure

There are several indicators possible. If the oil under the oil filler cap, or the oil on the dipstick, has a milkshake-like appearance, this would indicate coolant leaking into the oil. A milky substance under the radiator cap would indicate oil in the coolant. White smoke coming from the tailpipe on start-up would indicate coolant getting into the combustion chamber.

Another more proven way is a coolant pressure test. Now the cooling system should never be opened or checked on a hot engine, or while system is pressurized, otherwise serious injury could occur. You may want to enlist the help of a mechanic, like one from YourMechanic, to inspect your cooling system, and replace the head gasket if necessary