This is not a question that I can answer easy because it is so general. Mental illness is recognized in many ways depending on the illness. What I will do is list some general signs for you.
Some of the signs to look for are as follows. Look for isolation. People with mental illness often isolate themselves from their family and friends. People who can normally concentrate well will find themselves distracted easily, and lack the ability to focus on simple tasks. Eyes are very important. Alot of people with mental illness have a very far away look, and they don't maintain eye contact very well. Some people will talk to themselves. Some people will start to talk about brand new people that no one except them have heard of. Alot of time people's appearance will seem disheveled. They will wear the same clothes over and over. Their personal hygiene often becomes very concerning, and eating habits will change drasticly. These are a handful of the things to look for when trying to recognize mental illness.
Treating mental illness includes therapy, psychiatric visits and medication. The individual who understands that they have an illness will reach recovery faster than someone who doesn't know, or doesn't believe there is anything wrong.
I hope this helps a little bit. These were very general questions so I hope the generalized answers help.