The easiest way to diagnose this is to go to a radiator shop. They can test for exhaust gas in the coolant as suggested by Umai Naa!!. Bubbles in the coolant and steam out the exhaust are other signs. Generally a blown head gasket will pressurise the radiator/and the coolant level will go down and the temperature will go up.

You won't tell anything by looking at the coils but you might be able to suss something by looking at the condition/colour of the park plug electrodes. You don't necessarily have to have water in the oil with a blown head gasket. It depends where the gasket has failed.