I feel I can learn a few things by standing on the sidewalk and looking at the home.
Roof- I can generally get a feel for the quality of the roof. Look at the type of shingle is it an architectural or 3 tab. 3 tabs are usually older, if it is a 3 tab are the corners starting curl. I like to look at the ridge line, is there any sagging. If so there could be some structural issues.
I always keep my eye out for window AC units- then their either isn't central air or it isn't performing well. Depending on the age of the house its possible that there is an old boiler and radiators and no duct work. Getting well sized duct work to a second floor can be a real challenge and expense. Window units in conjunction with a condensing unit then I assume there is a ducted furnace that isn't working. A visible condensing unit can be an indicator of how old the furnace is.
My real fear with something that I'm not able to get into is the foundation. A bad foundation creates so many issue.
Just some of the things I like to pay special attention to