White smoke from the exhaust
Missing on a cylinder or 2
Loosing coolant but no leaks (or with leaks)
Overheating, but you have to KNOW for a fact that it’s full of coolant and there isn’t any air in the system.
Low compression in 1 or more cylinders usually #2 & 3
Water in oil or oil in water or both
Almost any one of these things can be another problem, but without doing some testing you won’t know.
Jeff