well a head gasket can blow in a few places
gasket to exterior of motor, coolant passage to oil passage, or combustion chamber to coolant passage
if your oil is turning into what we in the auto repair bizz call a 'milkshake' id be fairly certain it was a coolant passage to oil passage blow, only way to test is to check the fluids, and if they look literally like a milkshake then its prolly blown
to check for a combustion to coolant blow, you need to put a radiator pressure tester on the overflow bottle, and start the ski, watch the pressure as you bring the RPMs up, if the pressure in the cooling system rises as the RPMs rise, its blown here
but the only way to know for certain is to it teardown, youll need to have the block and head checked for cracks, head should be resurfaced before reassembling