The signs that you have a blown head gasket are white milky oil, white smoke coming from the exhaust, coolant loss in your radiator with no other coolant leaks.
You may also have coolant leaking externally from below the exhaust manifold or bubbles in your radiator or coolant overflow tank.
Fouled Spark Plugs, Overheating Engine and sometimes rough engine idling can happen due to a blown head gasket.
If the head gasket is not completely blown and has just a little bit of leak in it allowing the coolant to flow into the cylinders then using some head gasket sealer such as Bar's head gasket sealer poured into the radiator after you drain the coolant can help seal off the head gasket.
If the head gasket is more severely blown then you'll need to replace the entire head gaskets or head gasket which can be costly and be a hard job to do if you never replaced head gaskets before.
Sometimes it may be better to get another used engine for the car or another used car or another new vehicle.