Music club CDs are very easy to identify if you know what to look for.  The two largest music clubs, which incindentally are the only two I have found to leave a distinctive "mark" of some sort on their items, are Columbia House music club and BMG music club.  Most Columbia House CDs will have 1 of 2 indicators on the back insert- (a) There will either be the letters "CRC" printed somewhere, usually close to the bar code, or (b) There will be some kind of note in the fine print on the back insert signaling that the CD was manufactured by Columbia House.  BMG music club CDs always have the same indicator on the back insert - a small note which replaces the bar code with the BMG disc number, and a small note which will read something like "manufactured for BMG direct..." along with an address.