How to know if someone is mentally ill?

Mental Illness. Some conditions , like schizophrenia, a psychotic disorder, may be more easily detected due to hallucinations, delusional thought content or paranoia. Mood disorders (depression , bipolar) and anxiety disorders(panic attacks, phobia ) require more in depth evaluation. Medical conditions may present first as a mental illness. If a loved one concerns you, ask the family md for an evaluation.
Interesting question. If you are asking about yourself, please see your doctor for a proper diagnosis! i look for signs of distress such as depression, thoughts of suicide, mood changes such as "manic highs." I am concerned when people are hearing voices, having visual hallucinations. I look for people who are unable to take care of themselves, or exhibit violent behavior. Can be hard to tell if drugs/alcohol in mix!