Emulator with a monitor open probably.  Find the chunk of memory where the player variables are and watch those.   There are also a few sources floating around for various random players.   The problem is beyond channel EQs anything else has to be coded player specific to be displayed.

There is a good amount of file format documentation for some of these formats though, see here.   Lot of interesting things.

edit: I forgot, a bunch of the exotic players have editors out on the net.  Here's an example  Some players exported a packed version of the song but some can load straight back into the editor like MODs do.

What's the best way to see the inner workings of a song recorded in a proprietary music format? Some of these formats are just renamed MOD extensions, but the ones that have my interest use custom routines and cannot be read by any tracker I know of.