Yours is a very good question Bruce. When starting out I too wondered what to look for when viewing originals.
Eventually, after attending a couple of shows, and still struggling with building (still happening...) I began making lists of the things I wanted a closer look at prior to attending a show. Having a list with me helped me with focusing on those details of a piece I might have missed otherwise. Rich is correct about soaking up the overall architecture of those guns you have an interest in, trying to digest the overall picture along with the major transitions of important areas of the guns. Pictures in books etc. often can't depict the nuances of those sorts of details and that is the reason viewing originals is revealing and educational.
As is said "The devil is in the details," having a list with you will aid you when at a show. View a piece at a distance first, try and answer the question "Why does this gun appeal to me, what about it has me wanting to examine it closer?" Approach the gun and (always) ask if you may handle it. If the answer is yes then do so, get a feel for its heft, and how it points, look at those architectural details Rich mentioned. Soak it all up. Now get out your list and begin answering those questions you brought with you. Doing that will probably add more questions to your list, and so it goes. Plus, a real bonus, at shows you will encounter contemporary builders of some note, so ask questions of them also, in my experience they enjoy sharing their knowledge. You are now on the road to learning a great deal about those old guns and how their makers solved the same problems you, as a builder, will very likely encounter.
Enjoy the journey.
dave