You can do a compression check with the intake manifold off. Heck, you can drive the car with the intake manifold off.

Ideally you do a compression check on a warm engine. You'll get good information doing it cold but it won't be... uhh... ideal.

Depending on your compression tested, you can use an injector fitting or a glow plug fitting. Either way the fitting goes into the prechamber. The prechambers are in the head. There shouldn't be any holes in the block that lead to the combustion chamber. Likewise there shouldn't be other holes in the head that lead to the combustion chambers other than the prechambers and valves ports.

Sixto
93 300SD