Common symptoms of a bad gasket are; white smoke is emitted from the exhaust pipe, bubbles in the radiator, overheating of the engine, milky white oil and significant loss of coolant without visible leaks. Most of the symptoms are internal with coolant leaking from the head gasket being the only external sign.
Having one of the signs of a bad gasket is usually not enough to prove a failure of a gasket, but multiple symptoms are most likely the result of gasket failure. It is advisable to have the vehicle driven as little as possible whenever the symptoms occur. Depending on the symptoms, either have the gasket replaced or repaired.
The cost of replacing a gasket usually varies from one mechanic to the other. Replacing a gasket is quite expensive in modern engines due to their location and the amount of labor that is required to replace them.
To prevent gasket problems; replace the engine coolant before pH falls below 7.0 and always pre-mix the coolant and water before putting them in the engine. Contact a cooling specialist to diagnose and resolve gasket problems. It is risky for one to do it on their own as they can damage the engine.