Seeing as sometimes water can end up in oil sometimes with oil cooler failure, is there a way to know for definate whether a failure is oil cooler or HG?
As far as I can see all of these will point conclusively to head gasket problem - and not oil cooler issues.
1. The obvious thing to do is a cylinder compression test, looking for a single cylinder or adjacent pair with lower compression than the rest. A dry-and-wet test will confirm that any low compression isn’t the result of worn piston rings (though theoretically under rare circumstances you could have an oil cooler problem AND an unrelated leaky valve, which will look like a blown head gasket...).
2. The advice usually given by Car Mechanics Magazine is to ask an MOT testing station to lower a gas analyzer into the expansion tank (not actually touching the liquid) while the engine is running, any sign of exhaust gasses detected will indicate a definite head gasket failure.
3. I noticed in the past that when a head gasket problem becomes severe (that was on a 4-pot 8v TC Alfa-Romeo unit), if you place your hand over the exhaust you can feel condensation which is actually coolant droplets – and they will have a distinct smell of antifreeze. Also, you could sometimes smell the antifreeze on the spark plugs.