The standard test for brushed armateurs is to place them in an AC magnetic field and see if any of their turns are shorted by noticing how much the current changes in the magnetid field generator.
The machine is called a "growler" and it is made with a magnetic core having a gap large enough to lay an armateur in. When the gap is empty, very little current flows in the primary. When you lay a good armateur in the gap, the exciting current increases, and as you rotate the armateur to bring different sections of it into the magnetic field, the growler makes noises. If you place a shorted turn in the magnetic gap, the current goes WAY up.
Anyway...you need a motor/generator re-winding shop to find a growler.