Your question is like asking "what's the hardest part of learning to weld?" it varies per individual depending on your background developement. AWS has a standard for visual inspection of welds B1.11:2000, Guide for the Visual Examination of Welds also the Visual Inspection Workshop Reference Manual is a helpfull learning tool
If nothin' else I found this link to some DOT powerpoint on inspection
http://www.dot.state.ga.us/dot/construction/materials-research/Documents/ppt/vis_inspect/sld001.htm