I would recommend that you use a Opto Coupler. An Opto coupler consist of a LED (Light Emitting Diode) and a Photo sensitive transistor.
That will create a safety isolation between your motor that creates nasty electrical spikes and your precious Raspberry Pi.
Take a look at the Youtube video for more detailed info.
https://www.youtube.com/watch?v=pYENAGK8qH4
If you really want to measure the current then is a ACS712 (Current Hall sensor) and since the Raspberry Pi doesn't have a Analog input a ADS1115 (Analog to Digital converter) that has a I2C interface that Raspberry Pi understand.