“What are the warning signs to identify mental illness?”

You need not worry about the signs and symptoms, that you are not having, enjoy your life and do not worry about the problems you are not having.

Any serious sustained change from normal behavior could be a sign.

Well. There are so many kinds of mental illness, and they all have different signs. So it would be very difficult to go through all of them in one day.

As such, how you feel ; determines whether you have any mental illness in the first place. So, the short answer is; being evaluated by a Mental Health Clinician, preferably a Board Certified Psychiatrist; and if you are below 18 years of age, a Board Certified Child and Adolescent Psychiatrist; is the only way to categorize the signs that you are referring to.

That having been said, if you have any strange feeling about yourself or your environment around you, or feel sad and tearful most of the time, or have to drink alcohol or use cigarettes or drugs to be calm or to sleep, or feel paranoid or suspicious of the motive of others (without justification), or hear voices or see things, or have anger problems-to mention a few; then you definitely need to talk to an age appropriate Board Certified Psychiatrist, and he or she will tell you if you have any signs of mental illness or not.

Yes, there are some general signs and symptoms of mental illness:

1. Difficulty falling asleep
2. Recurrent nightmares
3. Decreased energy and loss of interest in things they used to be interested in
4. Restlessness
5. Anxiety attacks
6. A lot of fears which may or may not have a realistic basis
7. Unusual or bizarre conversations
8. Decreased performance in school or work
9. Thoughts of harming self or others
10. Mood swings
11. Angry outbursts and increased irritability

There are no reliable general signs, but be aware of any behavioral, personality changes or reduced ability to perform activities of daily living or perform in your job.

There are many forms of mental illness. General principles: falling off of role responsibilities, loss of interest in things, not showing up, suicidal thoughts, drunken stupor, stopping breathing and turning blue -- like I said, there are many illnesses.

Any distress that persists or is overwhelming, interfering with emotional state, social, occupational or academic function may benefit getting a check up, even if it's just with an attuned general practitioner. It doesn't mean medication is required, maybe not even therapy. There are a variety of approaches, exercise, sleep, diet, social activities, thyroid/medical considerations, and maybe nothing at all is required. Sometimes a sentinel event may suggest preventative care. My husband died and I put my 9yo in therapy even though she was coping amazingly well. It got in the way and she stopped going for 18 months, but we moved and I noticed some little things and got a good referral. She sees her therapist periodically as she wishes and it's not generally about her dad. Just a teen who doesn't always want to talk to mom. No harm done and hopefully she'll be in charge of where her life goes by understanding where her emotions come from and being able to direct her thoughts and behaviors in a constructive way.
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Mental illness robs your life of enjoyment and pleasure. Your energy and functional capacity are decreased. You should first see a good doctor to rule out a medical cause. Then get a referral to a well trained psychiatrist to make a diagnosis, then allow him to treat it. You will be a much happier successful person. Jan Fawcett, MD