We just bought a TCL 50" P8M, and it has certification for HDR10. How can we know if the video playing in YouTube is in HDR? In my phone (that can display HDR), it would say HDR next to the video quality. In the TV, there's only the quality and no HDR next to it. The picture preset is set to Smart HDR.
If your TV doesn't support the Logo Popping option for HDR content there's one way to identify other than using Smart Youtube.
While watching the video on youtube go to '"Stats for Nerds" option, if it shows the color as bt709 then it's Non-HDR but if it shows bt2020 then it's HDR.
Sideload the Smart Youtube TV APK
https://smartyoutubetv.github.io/
Search for an HDR video in the App...
Once the video starts playing, within the App you can select Quality, then Video and see which stream it's playing and change to one of the HDR streams.
I don't think YouTube does HDR yet