In my experience, it’s true that some are, but many are not. Many take their craft very seriously; they attend classes and dedicate hours to studying and perfecting their abilities, and ethics are important to them.
For example, I have had mediums who ask clients not to give any personal information when booking a reading, or who hire assistants to manage all bookings so they don’t know anything about the future client going into a reading. In group readings, some mediums have shared that they know some information about me “by normal means,” meaning they know I lost my dad ahead of time.
To avoid mediums who are fraudulent, I suggest staying away from the storefront psychic shops where they have signs offering cheap, $10 readings. I also suggest, as I mentioned above, concealing your information so mediums cannot “cheat” by looking you up beforehand. (The ethical ones won’t anyway, but if they end up giving you accurate information, this will ease any potential doubts about how they got it.)