Just like for physical illness, a blood test or imaging is done, for mental illness, psychological assessment is done. Psychological testing — also called psychological assessment — is the foundation of how psychologists better understand a person and their behavior. It is a process of problem solving for many professionals — to try and determine the core components of a person’s psychological or mental health problems, personality, IQ, or some other component. It is also a process that helps identifies not just weaknesses of a person, but also their strengths.
Some tests are used to determine IQ, others are used for personality, and still others for something else. Since so many different tests are available, it’s important to note that not all of them share the same research evidence for their use — some tests have a strong evidence base while others do not.
Psychological assessment is something that’s typically done in a formal manner only by a licensed psychologist. The duration of the test depends upon what kind of testing is being done. Testing is usually done in a psychologist’s office and consists largely of paper-and-pencil tests (nowadays often administered on a computer for ease-of-use).