The compare module comes with WeBWorK. It enables an instructor to view two pg problems. It renders both problems, and gives "diffs" of the source.
To enable the compare module on your server, edit the file webwork2/lib/WeBWorK/ContentGenerator.pm. Search for Compare; it appears in one commented line. Uncomment that line, and restart your server.
If you have the program "hdiff" on your source, you can get a colored diff of the source files. To enable this, search for hdiff in the file webwork2/lib/WeBWorK/ContentGenerator/Instructor/Compare.pm and change the value of $use_hdiff to 1.