How to detect feedback on motor
Measure the collector voltage. When the lock engages the apparent motor resistance will drop, current will rise, and so will the collector voltage. Of course, this assumes that the motor current rise does not pull down the Arduino 5 volts too much. 

Note also that, once you turn off the transistor, the collector voltage will rise to 5 volts. Dealing with that is a software issue, and is left as an exercise for the reader.