Agreed on the use of a pressure tester. You might need a special adapter to fit the coolant expansion tank.
But there's also something like this that directly checks for combustion gases in the coolant:
https://www.amazon.com/Lisle-75500-C.../dp/B0007ZDRUI
It would be a more direct test to rule out/in any head gasket problems.
It applies a vacuum to the coolant tank, and if any combustion gases are present (as they would be if you've got a head gasket issue), the test fluid in that little tube will change color.