To fully test the armature you need to remove it and use a bit of kit called a 'growler'. Most powertool service shops (especially manufacturer approved ones) will have one of these. You can get a fair idea form a couple of visual inspections too though.
It is best to remove the motor housing so you can see the rear of the armature clearly (the commutator). First of all though, before you completely remove the motor housing you will need to remove the brushes. The running faces of the brushes should be nice and smooth, if they are pitted and and rough looking across the entire running face then this shows that the motor has been sparking too much and the armature is most likely to be faulty. If the running faces of them is only pitted and dull halfway across this would indicate that the field coil is faulty and would need replacing. Once you have checked this, if you want to double check the armature, you should continue to remove the motor housing. the field coil will usually stay inside the housing when you remove it which is good as it means you don't have to disconnect any wires. You need to visually inspect the copper segments at the end of the armature (the commutator). All the segments should be of a similar colour and not too dark, although there will be carbon 'burnt' onto them. The gaps between each segment should be pretty much straight and all the same between each bar. If an armature is shorting out you will normally find a couple f the segments have started to raise slightly. These segments will have an abnormal gap between them and be darker or appear mire burned than the others. If they all seem fine, the only other check you can do on an armature is with a growler so you would have to get in touch with your nearest repair shop and see if they can help you.
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