Mental Illness Test Learn about how mental illness tests can help with diagnoses and treatment plans. Article Contents What Is Mental Illness? Undiagnosed mental illness can cause substantial stress and interfere with daily life. One in five people in the United States lives with a mental illness, accounting for 52.9 million people. Taking a mental illness test is a great step towards determining the scope of the disorder in order to guide you towards applicable treatment.1 A mental illness test is a psychological tool