You can also rent a combustion gas tester for free from Autozone (and stores like that) - just buy the tester fluid. It will test for combustion byproducts in the cooling system.
https://www.autozone.com/loan-a-tool...ter/391378_0_0
Let the bubbles go through the tester, if the fluid changes color, it's a bad HG.
When a HG gets bad enough, it's easy to tell. It's those marginal failures that can be harder to diagnose. Sometimes they only leak when it's hot. Or you have it floored. Or both.