How does one determine if a psychic or clairvoyant is real or a scam?

Making the assumption that there are people are a legitimate psychics and clairvoyants, how does one determine a real one from a fake? What are the criteria used to determine if a psychic is actually a psychic?
Update: While I tend to agree with Paul, I was hoping to elicit a response from a believer in this type of paranormal phenomena.
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