How are mood disorders diagnosed most often?

Psychiatric history. The usual way to diagnose mood disorders is with a good history and if there is some confusion, scales can be used to help with the diagnosis, but the bulk of ths diagnosis is made via history, that includes, family history, medical history, drug use history etc.
Mood disorders. By clinical evaluation for signs & symptoms of mood disorder, then running a test (depression or mood disorder scales) to determine extent & severity.