A, B, C. What you are looking for here are two things,
sound and heat, the motor should be more or less silent,
just a gentle whirring noise or whistle with virtually no
heat as it is not under load. If there is either of these it’s
usually a mechanically dead motor and needs replaced.
Fifth test
While still on the drill, again spin it up to full speed then
use your multimeter on the (highest range first if not an
autoranging meter then work your way down the ranges
until you see a reading) VAC setting, test the voltage
output across each phase in turn.To do this keep the
three wires isolated from each other labeled A, B, C.
Connect the meter to the bare wires of A and B. Take
note of the AC voltage. Then repeat this test on wires
labeled B and C then wires labeled A and C. All three
readings should be pretty much the same, providing your
drill is running at the same speed on all tests. If not, then
dead motor. Needs replaced.
Sixth test, no multimeter (optional)
If you do not have access to a multimeter (I strongly
suggest buying one) then you can try swapping out the
motor for the diagonally opposite one. E.g. Clockwise for
clockwise motor to see if this works. This means
