Sometimes when you talk to people, you can try a 1000 times to say something positive and they still bounce back with a negative statement. Having been through depression, I know that the illness wants to absorb any optimistic thoughts and turn them into whatever leads to the conclusion that you are no good and that there is no hope.
So how to differentiate between someone who moans and a person who is depressed?
One way is the rule of 3. Here's an example:
Your friend: "I went to Berlin last week but it rained all the time."
You: (#1) "But at least you got some time off."
Friend: "Yes, but we couldn't do anything because of the rain and I had to prepare work so there wasn't much point with the trip."
You: (#2) "Sorry to hear that. Hope you got a lot of work done?"
Friend: "I did, but I just hate my job."
You: (#3) "At least you know that you don't like it so you can start looking for something else."
If the other person responds with a negative statement after my 3 attempts, there's reason to consider that this person might be feeling low a large chunk of the time, maybe even depressed.
The other test I do is to listen if the person picks up on positive/general topics in group conversations. If the person only jumps in when it comes to challenges, complaints and criticism but not when it is about e.g. art, positive stories and appraisals, that could be a sign of depression.
So, to summarise, to bounce back with a negative comment, but being able to shift into positive feedback shows me that the person probably doesn't have a dark view of things. Similarly if someone can criticise as well as tell a positive story in the same conversation, the person is probably healthy.
Being depressed can feel like someone just pulled down the curtains and everything went dark. That's why when you are ill your response and engagement with others reflect your mental state that things are dark and hopeless.
I use the above tests on myself as well to check. If you feel like the friend in the examples above, it is time to seek help. Know that change is possible!