What is the best way to isolate a bad head gasket before tearing the motor down? I have oil in my coolant (or it could be a leak stop additive) and I have white vapor coming out the exhaust after the car has warmed up for 20 minutes or more (not condenaste). Not a lot of vapor but enough to wet your hand if you have it at 1/2 throttle. Is there a way to determine a bad head gasket vs a cracked block etc without taking the head off? Car has 124K miles. 2.3L motor.

I don't know the history of the car but it runs well and has good compression. I'm not getting coolant in the oil. I do have a leak in the heater control valve and the heater core.

I'm assuming a cracked block is a rarity with this motor but was wondering if someone has some experience with this motor.