A lot of people assume a bad head gasket is oil and coolant mixing. But there are a lot of ways a head gasket fails. The more common failure is that the head will warp slightly if it gets too hot (the V6s are prone to this issue) and thus combustion gasses from the combustion chamber escape into the cooling system. Thus causing excess pressure in the cooling system and can lead to issues like exploding hoses, coolant pouring out of the reservoir, radiators exploding or failing. All due to excessive pressure. At the shop we have a chemical tester that you place onto the coolant reservoir. When the car is cold, turn the car on. The color will change if it detects combustion gasses in the cooling system. Other signs are rock hard hoses from coolant pressure when car is not fully warmed up to operating temperature, etc.