Suicide is inherently difficult to predict. Many more people will show warning signs for possible suicide than will ever actually make an attempt. Not everyone who attempts suicide will fit a typical profile or communicate warning signs in advance. Warning signs for suicide can be obvious or so subtle as to be invisible. Signs may build up so gradually so that you don't notice them, or come on so quickly and intensely that you hardly have time to react. Faced with the difficulty of never being able to accurately know who is truly at significant risk of suicide and who is not, it is safest to assume that anyone who is showing any warning signs for suicide is at risk. Past behavior is the best predictor of future behavior. Therefore, anyone who has a history of past serious suicide attempts should be assumed to be a suicide risk now, particularly if any warning signs are present.
Mental health professionals tend to think about suicidal feelings and thoughts on a continuum (with high risk and immediate danger on one end, and low risk and little immediate danger on the other end). As a general rule, the level of danger suicidal people present to their own lives increases dramatically as they progress along the steps towards a suicidal act. This is to say, people's risk goes up as they move from 1) thinking about suicide (e.g., suicidal ideation), to 2) planning their suicide, to 3) collecting the necessary equipment, and then finally 4) actually trying to commit suicide. The earlier in this progression they can be identified and helped, the better.
Suicidal thoughts, commonly known as suicidal ideation, are the most important and most common warning sign indicating suicide risk. Although it cannot be known with certainty what others are thinking, anyone who regularly includes themes of suicide or death in conversation, writing, music or artwork (e.g., by talking about giving up on life, or how others would be better off without him or her) can be assumed to be sending out a warning sign that they are experiencing suicidal ideation.
If you suspect someone may be experiencing suicidal thoughts, go to the person and ask directly if he or she is thinking about killing him or herself. Don't beat around the bush. Don't be indirect. The topic of suicide is too important to risk remaining hidden. You're perhaps being nosy by asking such a direct and personal question, but you may also be the first person to notice the problem and be in a position to do something about it. Given the stakes involved with regard to suicidal behavior (i.e., a person's life!), it is better to be nosy than to be polite. It is far better to ask the question than to ignore your gut feelings and let a problem continue on to an unfortunate and lethal conclusion.
Take any suicidal talk (or any behavior that leads you to believe that someone is suicidal) seriously. You will find yourself overreacting at times when you do this. There will be "false positive" situations (incorrectly assuming people are truly suicidal when they are not) where you take someone's suicide joke seriously and end up making much ado about nothing. However, if you fail to take all occasions of suicidal talk and behavior seriously, you run the risk of missing or ignoring clues and warning signs that are leading up to an actual suicide gesture. The danger associated with failing to react to a real warning sign of suicide is so great as to make it quite acceptable for you to end up with a high rate of false positives.
If you ask, "Are you suicidal? Are you thinking about harming yourself?", and the person tells you "Yes, I am suicidal", then your next question should be, "Do you have a plan for killing yourself". This is the next logical question to ask because people who have a plan for killing themselves are at greater risk for actually harming themselves than people who doesn't. Further questions might be, "When are you planning to kill yourself?" and "Do you have access to the tools and equipment you need to kill yourself?." The sooner people intend to kill themselves, the more risk they present to themselves. If someone's risk is imminent (i.e., if he or she is ready to kill him or herself this evening) then get on the telephone with the emergency operator and do what you can to have this person hospitalized. It's better if you can talk the suicidal person into making the call, but if you can't do that, make the call anyway. Let a doctor or the police have the responsibility for keeping the suicidal person safe.
Some people think that bringing up the topic of suicide around people who are prone to such thoughts will encourage them to kill themselves. This isn't true. Your discussion of suicide will not trigger someone to act. Instead, your talking normally about the subject will convey your concern and may help cut through the shame that the person may have about talking about his or her suicidal feelings. Bringing up the subject of suicide is more likely to lessen the threat of actual suicide than to increase the likelihood of the event.
Other people avoid asking about suicide because they consider such talk an invasion of privacy. This might be true to some extent, but in this case, such privacy concerns are overrated. What good is privacy when someone is in danger of dying unnecessarily and before their time? Failing to ask about suicide out of some concern about embarrassing someone you care about might enable an unnecessary death, whereas asking about suicide could help save a life.