It's usually listed in the album credits in the original (or cover) version of a song.
Example: Go to ascap.com and type "Purple Rain" (with NO quotes!) in the search box. In this case, the song "Purple Rain" has two co-publishers: Controversy Music c/o Universal Music Publishing Group - and W B Music Corp c/o Warner Bros Inc
NOTE: We're forcing you to find these for your own good! We didn't NEED to be so strict about this.
The contract you signed said you had taken care of all of this already AND gotten permission from the publisher!
... but we realized a lot of people didn't actually get permission first, so we're trying to help keep you out of trouble.
The copyright police might not break down your door and stop you from burning a CD of you singing "Purple Rain", but they will find you much easier and even sue you if you released your version of "Purple Rain" without sending notice to Controversy Music c/o Universal Music Publishing Group - and W B Music Corp c/o Warner Bros Inc - and paying your 9.1 cents per copy sold.