To determine if it is functioning, open up the Performance Overlay to Level 3. It will say HDR ON in the top left info screen if it’s working. If it is not working, it will SAY NOTHING about HDR. Edit: Easier way, if it’s working it will have an HDR icon next to BRIGHTNESS in the quick settings.

Pro tip: Use Proton Experimental in game compatibility to get HDR working for some titles. It’s required for Doom Eternal and FF7R atm.