Most urban myths are, by their very nature, at least superficially believable. The organization that supposedly originated the message is not one you would expect to be involved in such things. If you are still unsure after trying these approaches, consider posting to alt.folklore.urban. But check the newsgroup's FAQ first. Many urban myths cause no material harm in and of themselves.
Some myths unjustly damage the reputation of a specific business or person.
Still others perpetuate hurtful stereotypes about how various groups of people behave. In general, you should think twice before forwarding suspicious messages. Forwarded e-mail can take on a life of its own. Don't Forward It for details about a message that has caused much grief yet continues to be forwarded. Some messages have an urgency about them that compels otherwise reasonable people to pass them on to everyone they know.