If you're in the field, you've heard the term "Practice Wisdom." It's used to describe the intuition clinicians develop through experience - a sixth sense or "third ear" that can't be learned in books and is hard to put into words.
It's interesting to note the differences between how professional organizations and third party payors have tried to "manualize" (i.e., standardize) treatment, and what experienced clinicians know and do in practice.
Take the Diagnostic and Statistical Manual of Mental Disorders, for example, soon coming out in its 5th edition. The purpose of the DSM is to standardize diagnosis so that we can communicate about clients (in research/academic arenas, and treatment collaborations) with some confidence that we're speaking a common language. But, the "standardized" diagnoses really vary widely - an individual has to have a minimum number of symptoms to qualify, but many different combinations of symptoms may all qualify for the same diagnosis.
In contrast, experienced clinicians recognize the "feel" of various diagnoses, and sometimes subtypes of commonly-combined symptoms. For example, depression can be characterized by neurovegatative slowing (increased sleep, increased appetite, psychomotor retardation, avolition, anergia, and so on), or the less common "agitated depression" (decreased sleep, decreased appetite, psychomotor agitation).
Even more tricky, how does one differentiate between psychosis that is part of a mood disorder (e.g., depression with psychotic features) and a primary psychosis (schizophrenia or schizoaffective disorder)? For me, it's a "feel" I get from the person: how present they feel in the room, how distressed they are by the psychosis, how aware they are of other people...and a certain look around the eyes.
How do you find yourself relying on specific practice wisdom to clarify what the DSM doesn't?