They show when you do a search.
If you're looking at a playlist, right click the file and select "Send To." Whichever option is greyed-out is where the file is.
Where else are you having trouble determining which files are in the Inbox and which are in the library? Everywhere else it's pretty self-explanatory (i.e. if you're looking at your library and you see the file, it's in the library / if you're looking at your inbox and you see the file, it's in the inbox).