Illegal file-sharers don't typically spend a lot of time designing their Web sites. Compared to a legitimate business site, theirs often suffer from a lot of rather basic and easy-to-spot errors.
Keep your eyes open for clear giveaways like these. The pages are filled with grammatical and spelling mistakes. Graphics are missing, improperly placed or totally out of context with the rest of the material. Fonts are ugly, weird, inconsistent or unreadable.
In short, the site looks and feels fishy and unprofessional -- and maybe just a little illegal.