You will need a reverse biased diode across the motor for back EMF suppression, such as 1N4004 Classic overcurrent detection scheme. The transistor is a PNP device. Resistor R is chosen so that at the current you want to detect, the voltage drop across it is enough to switch on the transistor, say about 0.6 volts. But to achieve this kind of resistance you can experiment with a small coil of copper wire, you won't need much.
Some commercial designs use a PCB track as the curent sense resistor.
Note that is circuit is not a precision design, the current at which it activates will vary a bit with temperature etc, but it should be OK just for detecting a stalling motor. When the motor stops rotating it'll stop making back EMF and the voltage across it will drop. You can use an op amp as a comparator. A Six-Legged Walking Robot. Lots of motor control info.