A bad armature will usually let itself be known with lots of sparks and some smoke from through the vent holes near the rear handle. If you tool has not done this and you still suspect a bad armature you will need to remove the armature and take it to a good repair shop. The shop should have some specialized tools they will use to test the armature. One test will put very high voltage through the armature. That will help isolate an open short in the windings. Another test will create a magnetic field around the armature and will find any shorts on the commutator.
I hope this helps with your repair!
-Mark
Review by a scott 03/22/2009
great tool. mounted an 8 " foam rubber "floater" disc for use as a non-digging sander. use stick on paper-cut my own discs from regular sheets- automotive adhesive or contact cement.