Compression check is a good 1st step. If it is inconclusive a leakdown test would be be a next step.
If you don't have a leakdown tester, you could try the following.
Remove the rocker shaft so all the valves are closed, remove all plugs, using the compressor tester cable, hook it to an air compressor and adjust it to an output of 20lbs or so then connect it to each cylinder one at a time. Then listen for air escaping from the radiator and other cylinders.
Try to do as much troubleshooting as possible with the engine still together as it'll be easier to isolate to a specific area/assembly. Hopefully it's just a head gasket issue...
Good luck
Dave
