A common way to test whether that's the case is to use
combustion leak detector to chemically probe your coolant fumes for oil and other impurities that get into it through gasket leaks.
My plan to find the leak this far is to rent a radiator pressure tester kit, but I will also look into
the combustion leak detector.
While
the combustion leak detector checks for carbon - dioxide (CO ₂) in the radiator.
I was browsing Amazon in search for
a combustion leak detector (AKA «block tester») and I stumbled upon a cylinder leakage tester kit, which is supposed to do similar things?
The usual means of testing is to take
a combustion leak detector (like this one) to the radiator.