Loss of compression will be seen in the engine being relatively easy to rotate with the glow plug installed. Remove the muffler, dripp 2 or 3 drops of fuel on the side of the piston just as the engine enters compression, and rotate slowly. You shouldn't see to much in the way of bubbles. A lot of bubble frming ndicates that the engine is starting to wear out the piston and liner set. (or the ring isn't seated in a ringed engine... and that MIGHT be cureable)

You can drip soap solution around the head to case joint, and rotate the engine. Rotate the engine a couple of revolutions. If bubbles form, you have blown the head gasket. A new gasket and properly retorqueing te head will often cure this. I'll usually get some automotive valve lapping compound and gently lap the head to the cylinder liner lip just to be sure of a good seal.

Soap solution at the glow plug too... if that's leaking you can just retorque the plug 90% of the time. Occasionally you'll find the copper gasket is bad... and I've seen ne case of the plug threads being stripped from over-torque. (dummy used a 3/8 drive ratchet and cranked for all he was worth....)