The module if failed is either open circuit or short circuit. Being that it is a semiconductor you can test each leg for one or the other. If you had the numbers written on the component we could have checked a cross reference book for the type of semiconductor. It may be a mosfet (metal oxide semi-conductor field effect transistor). How to test a mosfet youtube video The rest of your dimmer is a variable resister or potentiometer or rheostat. You can test that for a failure with a multimeter in the ohms setting, though it is less likely to fail that the mosfet.