This question should best be left to a professional in the mental health field. However, if I spotted the following behaviors I would try to get help for them.
An obvious change in personality.
Giving away objects that have meaning to the person.
Suspicions that the person is drinking to excess or using drugs illegally.
Suspicious cuts on the arms and legs
Shorter attention span than usual
Only a professional in the medical or mental health field can make a diagnosis of depression, anxiety or other disorder. I had a co-worker who suffered with depression for two years. He came to work and sat in his office and really did nothing. It turned out to be an illness called late life depression where there is not only an emotional factor involved, but the body stops producing certain substances that we need to maintain our emotional balance. He had a long journey to get back to his old self. Had he not tried to hide it, the problem may had been resolved sooner. But, I cannot emphasize it enough--if you suspect something seek out PROFESSIONAL GUIDANCE. Do not medicate with something you once took. The results could be fatal. Get help.