There are several ways a head gasket can blow - oil/water mix is just one of them. They can blow cylinder to cylinder, cylinder to outside, cylinder to sump (via the pushrod apertures), oil leak, water leak, cylinder to water jacket. If a cylinder blows to the outside, then you will hear it (a distinct chuffing noise). Running your hand around the head/block join will find it.
Cylinder to cylinder causes a significant power loss as the cylinders share compression. A compression test is the quickest way of determining compression loss in a particular cylinder.
Les.