Hi Tim.
This part of RDM was not well thought out, if at all.
I've run in to this problem before, the biggest problem is finding a way to implement this in a generic controller, see the link for the previous discussion.
http://www.rdmprotocol.org/forums/showthread.php?t=1063
http://www.rdmprotocol.org/forums/showthread.php?t=24
My approach is as you describe, test every possibility to determine what tests are available.
Hamish