I've been severely depressed before and I've been suicidal before. When I was severly depressed, I wasn't suicidal at all. I just cried all the time and showed the above signs. When I was suicidal, I was only mildly depressed. What triggered my suicidal feelings were a lack of hope NOT depression. So no, I didn't show any warning signs at all. Please keep an eye on your friends and family, especially when everything is not going well in their lives.
Another thing I'm wondering about. When people DO show obvious signs of depression/disturbed/suicidal behavior, and others do not acknowledge it, what is going on? Do they just not care? Or, are they inept/afraid of approaching these people?
I was at party a while ago, and this girl (and a friend at that time) was crying for like six hours straight over a guy. They weren't even in a serious relationship. Only me and one other guy was concerned about her. We seriously thought that she could've harmed herself. I didn't know what to do, but to keep an eye on her.
Same thing with a girl who was in my sorority's pledge class. This girl was obviously unstable, but most of the girls passed her behavior off as PMS.
No one really approached me about my problems either. Not only was I depressed at times, other times I was super happy, like euphoric. It was obvious that something wasn't right, when I go from from happy-go-lucky to sad and then back to really happy, like within a day or two. I was having mood swings. Fortunately, this ended when I took myself off the anti-depressants.
But, I guess it will be awkward to tell someone "I think you're having issues, you need to see a psychiatrist!"