A dead giveaway that a DVD is a bootleg is if the seller advertises that the DVD comes in a "color collector's case." That means a plain package. It also means "recorded off TV."
You don't want something that's been recorded off TV. Quality suffers.
Do your research. Check to see if the studio logo or other trademarks are on the box. The absence of these could be a sign it's a bootleg. Check IMDB.com or Amazon.com to see if the movie has been released on DVD. If it hasn't been released yet, what you're buying isn't legit.