In general don't do a full H&P for patients who are not brand new -- there's nothing more painful or irrelevant than hearing a student list a 6-day-old review of systems. Just do a SOAP-style presentation (although you should certainly try and know the full backstory).
As far as knowing what's pertinent, it should be part experience and part common-sense; pertinent positives/negatives are going to be different for every disease process and that part takes experience, but it's common sense to associate a disease with a particular set of body systems and highlight your presentation accordingly. A child with asthma for example, obviously highlight the respiratory system. Vitals (always vitals), breath sounds, presence/absence of retractions, chest X-ray findings, subjectively how the patient feels, etc. Not so important would be their MCV or %monocytes, right?
Later on the experience parts kicks in, and you'll know that steroid treatments for example cause certain things to happen in the labs and maybe on physical exam, and you can add this information to your presentation. You might learn the way we classify asthma and put it together with the child's history to really add something to the assessment/plan.
For now though, clues can come from several sources. The previous day's note should be your first go-to, as it should clearly spell out the pertinent stuff to be done that day. The next is a textbook; you should be looking up information on diseases processes at every opportunity, and this information should guide you in what to be looking for each day. Finally, run the case by your intern each morning. Students often uncover tidbits that we may have overlooked ('cause we're carrying 7 patients instead of just 2), and generally we can give you a little insight and some tips (Dr. X loves to ask about the difference between drugs y and z). That way everybody looks good and the patient gets better care.
