damscot:- the simplest metod to test armatures for shorts,open circiuts or high resistance joints is by means of a "drop test".all you require is a low voltage source (preferably dc) and a variable volt meter. the leads are placed about a third of the way around the commutater (a variac would help to reduce current)and measure the volt drop between segments a pattern should emerge.for open circiuts  no reading will be observed for shorts a higher reading will be found.once all the bars have been tested in the first third move the leads around the same distance and continue to test until all the bars on the comm have been tested.
hope this will help.
kind regds.