How do I make a game automatically detect external files?

I'm making a rhythm game and need help with creating the song selection menu.

What I want is for the game to include a "Songs" folder, which contains one folder for each playable song in the game. The game should detect each folder that contains a file named "SongData" and create a menu choice for each one.

Players should be able to add more folders to the "Songs" folder and have the extra songs be added to the menu automatically.