Copyright and how to tell
 

I'm sure that this question has been asked many times, but I'm still having troubles understanding. I've searched through many forums and I've come up with nothing.

I understand the LimeWire isn't illegal, but downloading copyrighted music is. How can you tell if the music you're downloading is copyrighted?

Also, almost all music is copyrighted anyways for legal purposes. Wouldn't that defeat the purpose of sharing music files if they're all copyrighted?

Anyways, thanks a bunch!!!

-KT