Two indicators that can alert you to potential trouble are the color of exhaust smoke and changes in the appearance of your oil when you check it. Exhaust gases from marine engines should be clear. Any color of smoke can warn you of potential trouble. Black smoke is the result of engine overload, a restricted air supply, or a malfunctioning fuel injector in the case of a diesel engine. Improperly burned particles of excess fuel are blown out the exhaust.
Blue smoke is formed by combustion of the engine's own lubricating oil.
This can be the result of worn piston rings, valve guides, or oil seals. The oil can come from an overfilled air filter in the case of a diesel engine or excess oil in the crankcase. White smoke indicates either water vapor from dirty fuel, a water leak into the cylinder or atomized, but completely unburned, fuel. Air in the fuel can also cause white smoke. You can not check the level and condition of your oil in your engine too often.