With your multimeter set for DC voltage, probe the solder connections circled in red while applying track AC power to the frame and center rail collector. The motor should spin and you should get a DC volt reading. Then using the direction button, press it to change direction of the motor and then again read the DC voltage.
Small alligator jumper wires can help with all of the connections. You can also remove the motor from the frame with the two screws on the opposite side (disconnecting it from the wheel gear train).
