Only to be used in a non live environment
As an alternative to ringing a transformer as described here, the following info describes a method to build very simple device to test a transformer winding for a shorted turn.
The transformer must be out of circuit i.e. no load. Connect the clips to the winding you wish to test, press the button ( only short duration tests). If there is no short, the neon bulb will flash brightly. If there is a shorted turn present, the bulb will either not light or light very dimly.
Try this on a few known good transformers to get used to how the neon behaves.
Neon Flashes= No Short
Neon Does Not Flash= There is a short