How do I know if the UV light is on Guardian AC4100

The model AC4100 does not have an LED indicator for the UV feature.  When the UV button is depressed and locked IN, the UV feature is ON.  When the button is sticking out, the UV is OFF.  You can test the unit manually as well. Please try the following troubleshooting:

1. Make sure the unit is plugged in and turned on to any fan setting with the UV button pushed IN.

2. Remove the back panel and filter.

3. There will be a small safety switch that the filter activates down at the bottom of the cavity where the filter sits (see picture below), it should be a small plastic or metal tab. Hold that down, the unit will power on.

4. When it's powered on you should see a blue glow coming from behind the metal shield. That will verify if the bulb is working or not.