are we able to spot dperession?

I was chatting with a group of friends one day.
I raised a question.

"How does one spot a person going through depression?"

Some of my friends gave some technical answers and some gave professional perspectives.

Until someone said this," You can see many smiles everyday. But you can never know whose world is actually upside down."

That statement embedded in my mind, Not the simplicity of the answer alone but rather the reality of this simple reply sent chill down my spine.

So is it time to rethink how we have been talking and connecting with people around us?
Do i bother enough to communicate deep with the person next to me>
Is that a way to invite a person out of his state progressively?