Using the manual priming lever at the injection pump once the air has been successfully removed from the injector you should be able to hear the injector pop off.
If your hearing impaired use an engine stethoscope with the probe placed on the injector body.
This procedure will work on many of the Lister and Petter engines, once you hear the injector pop off cease priming as you'll flood the cylinder with raw fuel. Making it difficult to start the engine.
In some instances a wooden dowel placed to the ear will assist in locating the source of an errant sound.