I hate to say it, but probably trial and error. I've downloaded some files that were marked as junk and they played fine. Check the file size. Most music files are over 2MBs(2000KBs). I stick with mp3's for music. You can preview it once it starts d/loading, and if it sits there forever awaiting sources it 's probably junk.
This thread has good info:
How to find music.
There's also a lot of fake files on the network. You can read more about them here:
Recognising Fake files.