A combustion to coolant air leak using the cylinder pressurizing method will make it's way to the rad fill point as bubbles or for a very slight leakage, the liquid level will rise at the rad fill point even if there is an air pocket somewhere. Just make sure to fill the cooling system to the top of the fill point and the cap is off.

The CO method of checking for a combustion to coolant leak is quick however if the leak is small it takes a while for the color to change. If the leak is large, bubbles will come from the cooling system more or less making the CO test redundant.

Head gaskets can fail in numerous ways, cylinder to cylinder with no coolant loss , cylinder to coolant , coolant to oil , coolant to outside of engine. Being generic and saying a head gasket failed for all engine faults is like saying the electrical system "has a short" when there really is an open circuit due to a broken wire.

I've been in and around the car business for 40+ years and have diagnosed / built countless engines, pressurizing each cylinder is the only way to test for this kind of leak.