A compression test will tell you which cylinder(s) have low compression (if any). There's a test called cylinder leakage, which is like a compression test with a PhD. It uses regulated air pressure on the cylinder to find where the compression is leaking to (rings, valves, cracks, head gasket).

Neither of the above will tell you if it's the valve guide seals.

There's a test for combustion products in the coolant, which is fairly cheap and easy to do, which will tell you if it's a head gasket or cracked head/block. You can buy the kit anywhere they sell Lisle tools. IIRC, it's about $50 or so for the kit, which is good for several tests.

My $ is still on the valve stem seals.