I agree with Andy - less than you can measure accurately. I've never
had to measure winding resistance in a small motor as I can always tell
what is wrong with it just by running it.

If your drill is burnt out, you will be able to see signs of
overheating on the windings. No need to measure anything.

By the fact that there is still some compression in the springs, I
think the brushes are probably not the problem. I have in the past put
a little piece of something between the spring and the brush just to
make sure just for a quick test. You could do the same - just make sure
the "something" can't fall into the motor housing and damage it.

Another thought - do your brushes have wires on them to connect them
electrically or do they just rely on the contact between brush and
holder?. If the former then I have seen the wire being too short or
caught round the back of the holder and thus preventing the brush
making good contact while still having plenty brush length. Again,
unlikely but something to check.

And Dave - interesting what you say about emery. I've never heard that.