Good questions. It's true that once a patient starts taking drugs to reduce anxiety or depression, symptoms of psychologic distress are less. This makes the symptoms harder to measure. The fact that treatment for a mood disorder is needed is a key factor. Before treatment began, the patient reported some symptoms that led to the diagnosis.
There are tests that psychologists use to measure psychologic distress. Most of these tests are in the form of a survey. The patient answers many questions. The questions often ask about mood, concentration, sleep patterns, and sexual activity.
Researchers have spent many years finding tests that will measure psychologic distress. Studies have been done to make sure that these tests are valid and reliable.