How do I know if I am watching Dolby Vision, Atmos, DTS, TrueHD, HDR10+ etc?

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Most clients will have a playback info option that looks like a little information icon or straight up says playback info. This will tell you whether it's direct play or transcoding, video and audio codec, and container. You'll also generally get limited additional information such as dynamic range (HDR/10/10+/DV) but no detailed info *most of the time*. The web clients tend to be the most verbose here (those that use the web player) and provide info like profile and level for the codec (but not DolbyVision).

For Atmos...honestly, you can tell. I'm pretty deaf and I can tell immediately when it's Atmos versus any other type of audio. And for audio tracks, if you have transcoding enabled on your server and for your particular user, selecting an incompatible track will trigger transcoding, meaning your server will convert the incompatible audio (or video in many cases) to a compatible codec on the fly. It's best to avoid this when possible but it's the whole reason Jellyfin exists; it's not always possible to avoid this scenario.