The signs that someone is about to commit suicide may be extremely easy or extremely hard to detect. Some people give no warning at all, while other's are explicit about their intentions. However, there are some things to watch for when regarding a potential suicide and these signs coincide somewhat with depression and other mental illnesses.
First of all, a great deal of the time, the person thinking about committing suicide will confide in a friend. They may do it in a joking way, but the intention behind their words may be dead serious. They may also express a general helplessness and feel that they have a lack of control of their lives. Thus, suicide is the only way they can control life.
If the person doesn't express their harmful thoughts verbally, looking at their behavior could provide clues as to what they are thinking. A good way to tell if someone is suicidal is looking at whether they have lost all pleasure in life. Things that they normally would find enjoyable, might bring them little to no pleasure. This is called anhedonia, and it is almost a sure sign that one is depressed, maybe depressed enough to commit suicide.
Also, a person may completely isolate themselves from others. For someone who used to socialize all of the time, this can be a dramatic change and should raise a warning flag. Any radical changes that occur in the person's behavior should raise a warning flag.
Furthermore, if a person gains or loses an excessive amount of weight in a short period of time, this could be a sign of mental illness which could lead to suicide. People who become depressed often lose their appetite, or begin to eat in excess in order to distract from their inner turmoil.
People who are suffering often will absorb themselves in their work or some other hobby, so much so that they don't do anything else. They attempt to distract themselves from the pain they are feeling and will completely engross themselves in whatever task they can.
This is not to say that everyone exhibiting these signs is contemplating depression. But, a great deal of suicidal people will show several of these signs at one time and it is important to recognize them before things are too serious. If you notice these signs in anyone and are quite certain that they are suicidal, it is imperative that they receive professional help immediately. Recommend that they go see their doctor, or go to a psychiatrist. There are things that can be done to prevent suicide, but it is not always possible as some people don't show any signs at all.
The important thing is to always be supportive to people who feel helpless and hopeless within their own lives. The will to live takes a lot of suffering to dampen, but it can be revived.