What are symptoms and signs of mental illness?

While everyone experiences sadness, anxiety, irritability, and moodiness at times, moods, thoughts, behaviors, or use of substances that interfere with a person's ability to function well physically, socially, at work, school, or home are characteristics of mental illness. Mental illness can have virtually any physical symptom associated with it, from insomnia, headaches, stomach upset to even paralysis. Socially, the person with a mental illness may avoid or have trouble making or keeping friends. Emotional problems can result in the person being unable to focus and therefore perform at work or school.

How is mental illness diagnosed?

There is no one test that definitively indicates that someone has a mental illness. Therefore, health-care practitioners diagnose a mental disorder by gathering comprehensive medical, family, and mental-health information. Patients tend to benefit when the professional takes into account their client's entire life and background. This includes but is not limited to the person's gender, sexual orientation, cultural, religious and ethnic background, and socioeconomic status. The symptom sufferer might be asked to fill out a self-test that the professional will review if the person being evaluated is able to complete it. The practitioner will also either perform a physical examination or request that the individual's primary-care doctor or other medical professional perform one. The medical examination will usually include lab tests to evaluate the person's general health and to explore whether or not the individual has a medical condition that might produce psychological symptoms.
In asking questions about mental-health symptoms, the counselor or other mental-health professional often explores if the individual suffers from hallucinations or delusions, depression and/or manic symptoms, anxiety, substance abuse, as well as some personality disorders (for example, schizotypal personality disorder) and developmental disorders (for example, autism spectrum disorders). Since some of the symptoms of any one mental disorder can also occur in other mental illnesses, the mental-health screening is to determine if the individual suffers from a psychotic depressive, bipolar, anxiety, behavioral, substance abuse, or personality disorder.
In addition to providing treatment that is appropriate to the diagnosis, determining the presence of mental illnesses that may co-occur (be comorbid) and addressing those issues is important when trying to improve the life of individuals with a mental illness. For example, people with schizophrenia are at increased risk of having a substance abuse, depressive, or anxiety disorder and of committing suicide.

http://www.medicinenet.com/mental_health_psychology/page4.htm