Several options. Psychiatrists (physicians specializing in mental disorders) are the traditional choice, but clinical psychologists, especially those with a phd or psyd degree, have extensive training as well, and are also trained to administer 'paper & pencil' tests for quantitative assessment of iq, memory, and many other factors. Any mental health professional can begin an assessment though.
Neuropsychologist. A medical physician, (md or do) can do some mental status assessments. Counselors, social workers, psychologists also can do some assessment. The most detailed evaluation is done by a phd degree holding psychologist trained and educated in the complex testing and interpretation of the results necessary for a neuropsychologic evaluation which can identify mental illness, learning problems, etc.
Assessment. Qualified mental health professionals from psychiatrists to psychologists to social workers to counselors, depending on the type of assessment needed.
Call your insurance. Contact your insurance company to find a psychologist or psychiatrist in your area who accepts your health insurance. If it is a serious emergency, go to the ER or call 911.
Assessment. A mental health assessment may be done by either a psychiatrist or a psychologist. The assessment can include an interview, observations, and even some standardized tests.