An alternative method is getting an oil sample checked.

I ran the combustion gas test on my Jeep 4.0 and it was fine. No evidence of a blown head gasket. The oil looked good too. Not milky.
I took an oil sample and sent it to Blackstone Labs. They identified high levels of potassium in the oil, which is from antifreeze. The oil analysis confirmed a head problem even when the combustion gas test didn't. Not a big leak, but enough to show high levels of lead in the oil, indicating the bearings were getting eaten by the antifreeze.