Diagnosing depression is highly subjective and qualitative. Generally, a patient must express depression symptoms (anhedonia, pessimism, negative thoughts, …) for >2 weeks. The secondary effects (including side effects) of taking antidepressants must also be considered, as they can be severe.
There is a genetic risk with all forms of depression, however it is not a single gene disorder and has a complex consistency. It is thought the genetic component is around 40% of the risk factor.