Okay, so I have created a few libraries now, and some of them have conflicts.

For instance, I have Library1. It contains a modified ShowRoomDescription() function.

I also have Library42, which contains a different mod of the ShowRoomDescription() function.

Is there anything I can do to provide a warning (or something) besides putting notes in the library files?

What do the two ShowRoomDescription functions do? The best solution would be to have them test if the other library is present (test if an object exists maybe) and act accordingly, so it would not matter if both are present or what order they are in.

ETA: If you have commands or turnscripts in your libraries, you can test if they are present using GetObject, which would be better than adding an object just for that purpose.

does quest have any feature to test library files?

such as with some programming languages having:

#ifndef NAME_OF_LIBRARY_FILE
  #define NAME_OF_LIBRARY_FILE
#endif

or can quest only test Objects (which includes Commands and Turnscripts, as you pointed out already in your post)