compression testing

[gsx600]

Hi there, anyone know if it's possible to compression test the engine out of the car ? I have a fly wheel and starter on the engine, but when apply power to the starter motor directly, only the motor turns with out engaging the flywheel.

Probably something stupid like pre engaged starter that needs another power source to energise the motor ?

Engine a late Rover V8, and wanted to test just in case problems before swapping

Ta