This is exactly what therapists go to school for years and years for. There is a huge difference between being "clinically depressed" and "situationally depressed." That is what therapists are skilled in discerning.

There is also a difference between "mourning a loved one" and "complicated bereavement." It depends how much your daily life is impaired by your depression, for how long, and to what extent. There is even a book of rules (called the DSM IV) that lists out the criteria for diagnosing people. I'm sure you've heard of it.