dc motor overload
Depending on your current requirements this might not be a so easy task.
I will suppose that you need to work with low currents, motors of less that approximatetly 200W.
You can use a small Ohmic value series resistor as other posters suggest and a transistor. With this method you might even enable a circuit that could interrupt the motor supply if you wish.
Another simple method is to use a series self wound coil arround a reed switch. You need to experiment with this method in order to find the number of turn of the coil. When enough current passes through the coil, the reed SW is energised and you can use its contacts anyway you like, i.e. drive an indicator, interrupt the motor's power or else.