Hi,
(You DID read this, right?) : http://arduino.cc/en/Tutorial/x10
There doesn't seem to be a standard or guideline for this issue. (Anyone have pointers?)
Many nicely done libraries have an information page or if on GitHub etc, a WIKI that starts with How-To.
But many times all you can do is read the .H file in wordpad etc. and look for stuff like:
CommandCode { cmdOn, cmdOff, cmdBright, cmdDim }; Telling you the possible commands.
More gory is to read the .cpp files and figure out by the coding what the author meant to do.