My apologies for responding to a thread a month old. I'm not sure if there are rules about that here or not.
As someone who has suffered from depression, is married to someone who has, and has had more than one friend who's suffered from it, I can say that it can be very hard to tell, as an outsider, when someone is depressed. A depressed individual, especially one who's naturally introverted, will be very good at hiding their depression. If you didn't know the person before their depression hit it can be even more difficult.
If you knew the person before, some signs to look for are:
1) Withdrawing from activities you know that they enjoyed
2) Withdrawing from seeing people they love and participating in group events
3) Excessive sleeping
4) Higher likelihood of participating in self-destructive activities (drinking, drugs, gambling, cheating)
5) Getting upset over trivial things
There are a lot more, for sure, like a generally negative attitude, but if someone displays the above over a time then something is probably up. Simply being negative or complaining isn't enough to show depression, but it can lead you to looking for other signs.
Also please understand that you can't help a person who does not themselves wish to be helped. Getting out of a depression requires buy in from the depressed individual. If they are not active in their recovery it's not going to happen.
As stress is a big part of what can cause depression, I've found that taking a test such as the one found below can open your eyes. There are so many stressful events that can contribute to mental health issues that no one thinks about. Buying a car, graduation, starting a new job, personal achievements, Christmas season... all of these things which normally might be thought of as positive things in a persons life add to your level of stress. During the height of my depression I took a test like this, and anything over 150 was stressed, over 300 was extremely stressed. I was 450. My wife was 600.
Sometimes it takes someone pointing out the little things.
But I digress. What's important is that if you fear someone is suffering you be there for them and try to steer them towards becoming healthy. Everyone is different and will react differently.
Online stress test: http://www.healthyplace.com/psychological-tests/online-stress-test/