I have not encountered anyone who was pretending to be a doctor or psychiatrist, fortunately, but of course we've all heard stories of people who do just that, and obviously some people have had to deal with such dishonesty.
With that in mind, everyone needs to remember that there are licensing boards in every state for these professions. Before seeing any professional, simply contact the licensing board for that particular profession and check up on the supposed "professional" you wish to see. While you're checking up on whether or not their license is current (or if they've ever even had one), you can also check out if there have been any complaints against them.
You might be surprised what you find out (although I hope you won't be), but in this day and age it pays to be proactive about these sorts of things.