There are a number of tests for a head gasket. Compression test, vacuum test, hydrocarbon test, inspection of the spark plugs and engine oil, cooling system pressure test, etc.
Any or all of them should be used to eliminate any guessing on this problem.
Coolant has a sweet taste but it's very debateable as to whether it could be tasted while mixed with motor oil.
I hope you're aware that coolant can sickness or even death in quantity and used motor oil can cause cancer.