There is no simple way to test it. It just depends on what is wrong. Just to make sure, the fog lights don't turn on right?
The easy test is to turn on the lights including the fogs, pull the foglamp relay and jump pins 30 and 87 in the PDC. If your lights are bright, you'll have to test the coil side.
This is more difficult, you need a jumper wire and a multimeter that can handle at least half an amp. Turn on the lights like before and jump pin 86 from the PDC to pin 86 on the relay. Next take your multimeter and put one probe in pin 85 on the PDC and the other on pin 85 on the Relay. If it buzzes record the current draw. Next, remove the multimeter probe from the PDC and place it on battery negative. If the problem stops, its likely an issue with your headlamp switch.
If it continues, remove the relay and jumper then take a test lamp and probe from battery negative to pin 86 on the PDC. The test lamp should light brightly, if not, and the high beams work fine, you might be looking at a multifunction switch issue.
