How do you know if someone has mental illness?

if they speak to themselves.
It can be even more subtle than that. It depends on the mental disorder they have. They could become withdrawn, quite, suddenly angry or sad, and so on. If anyone shows any of these signs it is best to get them help. It could always be something simple. But if not it could lead to suicide or drugs or even alcohol. There are many sites you can go to find the answers you may be looking for. However, when in doubt consult a physician in the Mental Health field.