You should look at API NAFSlist_dir(). It is used in conjunction with API NAFSdir_entry_count(). NAFSdir_entry_count returns the number of entries available. This is passed to NAFSlist_dir so that NAFSlist_dir knows how much space to allocate.
These are described in the API reference guide at:
System Software Services->File System->Flash File System->Functions.
Please ignore the fact that this is listed under Flash File System, it works equally well with the FLASH file and the RAM file system.