Normally, the resistor on which you would measure the voltage drop is very small. This leads to the conclusion that the voltage also is going to be very low.

1. Put the resistor in reference with the ground. I mean Gnd-resistor-Motor+Vcc

2. On the resistor you have to filter and amplify the voltage with an op-amp to bring it to a level enough for the PIC to measure it.

Ioannis