Head Gasket,Cracked head or hole in piston

First pull all the spark plugs, you should be able to see which cylinder or cylinders it is. The pull that head off, if its the head gasket going you should be able to see the damage. If the gasket looks good, then get the head check out, if its fine, then you need to pull the pistons out because the rings must be damage.

Thats how I would go about it, you have a problem that needs to be repaired, why waste time running a compression test ?