[ashlar]

But I have to exit out of the film to do that. I know what is running, what I don't know is what decoder it is actively using.

No. If you access Codec Info (O on the keyboard with standard keymap) you get the list of codecs used. LAV filters followed by (i) mean that Kodi is using the internal ones of DSPlayer.
Once you are sure of that, you press M to bring up the menu, then you select Video Settings from the on screen menu (all this while the video is playing), you go to Filter Lists, you select the filter you want info on and its config window will appear, just as, in MPC-HC, doing right-click/filters/filter's name.