Start from far, far away with half-closed eyes and an open mind. Pay attention to what people are telling you. Focus broadly at first, assessing people's health and status, then probe more deeply into specific systems.
Once you have some idea of what is happening, go about testing your idea. This means knowing diseases and conditions, their signs and symptoms, and investigations to do (including questions) to rule things in or out. Avoid too narrow a focus from an "oh, I found it" attitude.
There are a number of acronyms to assist with differential diagnosis:
Be aware of the most serious conditions and rule them out first, using history/physical and other investigations.
It is important to know how to quickly screen for conditions. Memorize basic tests, such as the MMSE.
Symptoms - pain, cough, fatigue, anxiety, depression - are not a bad thing. Instead, they are a sign, pointing the way to something that is happening.
Cognitive and emotional bias can have a pround role in having us reach a proper diagnosis.
There are many electronic devices to assist in making a diagnosis. This reality needs to be taught (Graber, Thompkins, and Holland, 2009).
Issues include overconfidence, tunnel vision, bias, prejudice, and inconsistency.
Diagnostic failure often results from issues in individual thinking (Graber, Franklin, and Gordon, 2005).
The process of reaching a proper diagnosis is extremely complex and in many ways invisible.
Medical students are often lacking in critical-thinking skills.
Graber ML, Tompkins D, Holland JJ. 2009. Resources medical students use to derive a differential diagnosis. Med Teach. 31(6):522-7.
Graber ML, Franklin N, Gordon R. 2005. Diagnostic error in internal medicine. Arch Intern Med. 165(13):1493-9.