While I prefer to use the diode test functionality of a DVM you can do a quick and dirty test with a 9V battery and a 12V light bulb. I grabbed an LED bulb but I won't guarantee all LED bulbs will fire at sub 9V. You will get a dimly lit 12V bulb with a 9V battery but the test works. I only used the LED because it showed up better with the picture.
If you first connect your bulb direct to the 9V battery then test it through the isolator you will notice a change in brightness. This is caused by the voltage drop across the diodes and is 100% normal and should be expected.
Wire the 9V battery and light bulb as shown and the bulb should light if the diodes are good..