I also agree that doing something is a good idea - but I wouldn't use the word "depressed" as that has a lot of negative connotations. I know when I was younger I thought of depressed people as having mental illness and mental illness is somehow bad and evil. Maybe something like:

"Excuse me [Insert Name], I've noticed that you might have something on your mind. I'm not asking you to tell me anything, and feel free to tell me to butt out, but if you ever need someone to talk too, I will listen. That's all."

So you avoid a lot of judgment, but you also invite them to MAKE the decision, instead of inserting yourself into the situation you provide yourself as a shelter from that situation. It's an invitation instead of an interrogation. If they say "No" or tell you off, just smile and say "I understand completely. If you change your mind, the offer is always open."

That's how I would do it.