At least with AMD GPU, an easy way to do it is to check usage of VCN.

Something like this (assuming your GPU index is 0):

    watch -n 1 sudo cat /sys/kernel/debug/dri/0/amdgpu_pm_info

You should see there:

    VCN: Enabled

If GPU accelerated video is being played.

I think you might need to set this flag to true in Firefox's about:config (or it might be not needed anymore):

    media.ffmpeg.vaapi.enabled