A lot of depressed people can hide it well and appear to be fine so you can't really tell with some people.
I would say the stereotypical depressed person would be someone who; never goes out, never laughs or smiles, always negative about everything, not a joy to be around, refuses help all the time and sometimes openly suicidal.
I would want to befriend someone in need so I can help them, however, some people are so deep in depression, it's close to impossible for them to recover. You can help them as much as you want but they'll refuse it and not take in the advice. So I don't think I'd start a friendship with a depressed person, for the sake of my own sanity.
I probably wouldn't date a depressed person. I've heard of people dating those who are depressed and it never lasts because the depressed person can start getting angry and upset with their partner a lot due to their emotional instability. It causes a lot of tension and arguments. Also, I want to be with a positive, motivated person who can make me a better person, not someone who's always negative and has practically given up on their life, their future and those around them. It's just not a nice atmosphere to be in personally.