I think a lot of people think "how could they do that and leave a wife,mother ,sister or whoever behind" but a depressed person isn't thinking of that in their dire times.
I know a person who has had many many thoughts of that dark road ahead, but the next step of converting thoughts into action is maybe the area that we need to understand how or if it will happen ?
As silly as this may seem it is sometimes a combination of a spur of the moment decision and the where with all to do it. Access to a weapon, dark thoughts whilst at work in comparison to, dark thoughts, no weapon and being at home.
Can you pick a suicidal person, I don’t believe so, the only real indicators you may see is if the effected person opens up, admits they are having some issues and seeks help.
It is important to allow people to vent sometimes, listen, offer help,let people know you are therefor them and encourage them to seek professional help