I think that as a friend, you should always mention it. This is what your role as a friend must be. I am a big fan of the work done by Beyond Blue and if you want to cross check your friends symptoms - here's what depression symptoms you might be observing: http://www.beyondblue.org.au/the-facts/depression/signs-and-symptoms.
There's also a fact sheet on what to do if your friend has depression and how to care for them: http://www.beyondblue.org.au/resources/family-and-friends/caring-for-someone-with-depression-or-anxiety
Regrettably, the first step is always the hardest, and getting your friend to realise what they might be going to can be very difficult. However, depression, like many illnesses should not go untreated, and whilst the first step is difficult, it is a step in the direction of recovery.
Sitting down with your friend, away from the hussle and bussle of life, and asking them how they are, can be a great way to find out how they are doing. If they don't open up, try again at a different time but assert to them that you are 'here' for them. Monitor the situation, and if you see signs that alarm you perhaps signalling self harm, or the lowest self worth (suicidal), then it's time to call the professionals, and some of these numbers may help: http://www.beyondblue.org.au/get-support/national-help-lines-and-websites
Good luck to you and your friend. From personal experience of helping someone with depression it's a difficult time, but just being a friend will make all the difference.
Thank you for the relevant links.
And I will look out for any indication of self harm.
Monitor the situation as you suggested.Simply support them by being a friend.