Recently, this paper  claiming to be able to distinguish bipolar disorder from major depressive disorder via a urine sample has come to my attention.
Despite reading the paper, I'm unable to understand how they discovered this bio-marker in the first place. What is the general procedure for identifying and verifying bio-markers in the domain of clinical psychology?
 "Divergent Urinary Metabolic Phenotypes between Major Depressive Disorder and Bipolar Disorder Identified by a Combined GC–MS and NMR Spectroscopic Metabonomic Approach" by Chen et. al