Some disorders can be diagnosed with medical tests. PTSD is the best example. Google PTSD and brain scans and there are tons of links to read about the research on changes and damage in the brain and how it is being picked up on brain scans. It is amazing stuff!

Other conditions are harder to diagnose. The DSM provides "criteria" for a disorder, but it is really only a guide. I personally think that pdocs and therapists rely on their experience more than anything.