There are a wide variety of mental health professionals, with an alphabet soup of letters after their names. This includes an MSW (Master of Social Work) a CMSW (Clinical Master of Social Work) the latter trained in doing therapy.
There is an LPC (Licensed Professional Counselor) and an LMFT (Licensed Marital and Family Counselor.) (I have an LPC and an LMFT.) They all have at least a master's degree, of various names, such as M.S. (master of science) M.Ed. (master of education) and various other types.
A psychologist has a Ph.D. and is more highly trained in doing research and in testing. He is not allowed in many states to call himself a psychologist unless he is licensed to do therapy. There is also a Psy.D. who has a doctorate, but has not done a dissertation, is less trained in research and testing, but better trained in doing therapy. The Psy.D. may advertise himself as a "mental health practitioner," and may have an LPC or an LBP (licensed behavioral practitioner.)
In Oklahoma, people who are not psychologists are not allowed to use words like psychology, psychologist, or psychological in connection with their names or describing the work they do.
There are industrial psychologists, who determine the qualifications that make a person best suited to do a particular job, what conditions allow people to do the best job, and who test people to determine their qualifications for a particular job. They do not do therapy. In some states they must refer to themselves with another title if they are not licensed.
There are also sports psychologists, who help athletes to best train for maximum performance. They do not do psychotherapy. They may be required to call themselves by another title if they are not licensed.
There are school psychologists, who do testing and make recommendations for childrens' education. They do not do therapy.
Some states have employees called "psychologists," who otherwise would not be. That is just a job title for them. Only a state or the federal government can legally do this.
There are fakers in this business, some of whom have a high school diploma, and others who have been candidates for licensure, who failed to get licensed because they could not pass the test, have not gone under supervision (required) or have let too much time go by before getting licensed, but continue to practice anyway.
They should be more than willing to show you a license hanging on the wall, or a card in their wallet or purse, showing that they are actually licensed therapists. Unless they practice in several offices, they will probably have their diploma and license on the wall, as well.
A psychiatrist is an MD, and can prescribe medications. If he is board certified, that means he has gone through his psychiatric training, has passed his tests, and has sat before a board and orally answered their questions. If he is board qualified, that means he has done everything except passing his board examination. A candidate is allowed to attempt to pass his boards several times.
In some states, including Oklahoma and Texas, any MD can volunteer to be a psychiatrist, and he is one, just like that, with no psychiatric training. Sometimes a psychiatrist who is not board certified has done his training and passed his tests, but has not yet sat before a board. In other cases, he has sat before the board, but has failed to receive certification.