How do I know if I'm downloading a legal or illegal mp3?

If you are paying for it then it's legal. To save the hassle and be safe, use iTunes because they are licensed mp3 files.

Rhapsody is the same thing but their push is they let you listen to the whole song before downloading. I would stick to well known mp3 downloaders like Itunes and Rhapsody rather then getting into the mess of the thousands of other mp3 downloading sites that may or may not be legal.

For more information on how to decipher between mp3 legal issues check out http://www.mp3licensing.com/