You can test the armature for a ground with an ohmmeter- check for continuity between the commutator and the shaft or core. A battery-powered test light will work in place of an ohmmeter. Continuity (or light) indicates a ground. A short in the windings (between commutator bars) is what shows up with a growler. There are possibilitess for testing for a winding short, but I don't think any of them are especially reliable, and probably more trouble than they are worth- so skip those. Any generator or motor rebuilder should have a growler.

Harvester