First of all, the resistor in series with a motor to control the current is not a good idea. At least it should be a very small motor or an effect resistor, but still not preferable. In order to control the current through the motor you should use a transistor/MOSFET. Are you trying to control the speed or the torque?
You should measure the motor current by directly supply the motor from the power supply, BUT BE AWARE that the small power supply current rating may be way to small. A motor, depending the size of course, will draw as much as 40 times the nominal current at start-up for a short time. The power supply must therefore be fitted with a slow fuse to match the inrush current.
Your power supply can supply 500 mA up to 12 VDC. If you draw more, the voltage will drop and if you draw a very higher current, the fuse will blow (if it's not just a cheap wall transformer)