This article is from the Anime Music FAQ, by Ru Igarashi with numerous contributions by others.
If you are buying CDs off the shelf, CDs from these companies are clearly labelled as "Son May", "SM CD", or "Ever Anime", so they aren't difficult to identify. But when online shopping, it is always a good idea to do a little research beforehand. You can get catalog information from CD information sites and online CD stores in Japan. If you forget the actual number, knowing which company released the CD and the prefix they use can help a lot. The more common ones are listed with the next few questions.
There is a good site for more general anime bootleg information at http://www.digital.anime.org.uk/piratefaq.html There you can find images of the SM and Ever Anime logos, as well as other anime piracy information.
Another decent general anime bootleg information site is http://members.austarmetro.com.au/~mwhitley/bootleg.htm It's focus is more on the Australian market, but the bootleg info is relevant.
 
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