That's a cop-out. It's not impossible, it may not be easy. People that say, "I had no idea... Would never have guessed in a million years that... ", after the fact, are giving themselves an "out" for being so self-centered, for not really caring. It's being concerned, interested, involved, obsevant of the day to day changes and nuances in words and actions of the people you work with or interact with daily. You can make a conclusion and enlist other co-workers to observe, confirm and approach these people with the suggestion to allow someone, maybe you, to help them.
Something else to keep in mind. Very often when a person removes themselves from this world, you'll hear people close to them say after the fact something like, "But (s)he was doing so much better!" or "But I saw him/her a few days beforehand, and (s)he seemed so happy!" It's not uncommon for the person in trouble to be in good spirits in the space between when they've finally made the decision to take their own life and when they actually do it. That's because they know that the end is coming, and the personal torment that they're suffering will soon be over. They may feel a sense of calm that comes from making such a monumental decision and knowing that they won't have to suffer much longer. So even if a person is being watched closely, it's easy to mistake their suddenly calm smile for a sign that things are getting better and the need to worry is over, when the very opposite is true.
Never forget, depression is an altered state of mind. Even if you can identify it in someone else, you can't fix it, because your reality doesn't apply to their reality. Depression is a liar, and a very convincing one at that. You can tell someone over and over again that they're loved, that they're worthy, that they're bleeping Superman, and they can still feel like the biggest piece of crap in the world if their depression speaks louder and more forcefully than you. And it usually does. Depression makes you feel like you're not worthy of the love being piled on you, that these people are just taking pity on you or blowing
smoke up your ass, and if these people only knew what a terrible person you are, they would never say those nice things to you. Depression. Lies.
And here's where I'm going to get a little brutal. To say, "Oh, well if
I notice something in a co-worker
I'll step in and save them," is narcissistic as crap. I suffer from depression from time to time, and I assure you, if it was as easy to combat as getting my co-workers to say nice things to me, I wouldn't spend so much money on therapy. Does that mean that you shouldn't say anything if you feel like someone is struggling? Absolutely not. Everyone's depression is a little different, and those kind words and support may be a moment of brightness that they can hang on to. But to say that co-workers, teachers, friends,
etc are at fault when a person takes their life because they weren't, (may I quote,) "being concerned, interested, involved, obsevant of the day to day changes and nuances in words and actions of the people you work with or interact with daily," is to say that they were capable of saving that person and were too "self-centered" to do so. And that's not fair. A person can be in therapy, can have a loving spouse and family, and have friends telling them constantly that they're loved, and still feel unworthy of breath.
So yes, be supportive. Be observant. Reach out with a shoulder and an ear when you feel like someone is hurting. But also know that you can't fight their demons for them, and for some, the fight will be just too hard to win.