A device for testing the magnetic properties of coins in coin operated apparatus such as parking meters, used in conjunction with a coin diameter testing device. The device includes a double arm rocker lever provided on a side wall of a coin guiding duct. A first arm of the rocker lever is equipped with a permanent magnet and the latter extends through an opening of the side wall into the range of a coin which has been inserted into the coin guiding duct and which temporarily rests on a coil support. A second arm of the rocker lever includes a locking bracket which can be controlled so as to be in and out of engagement with a testing segment required for the test for determining the diameter of coins.