The first thing that springs to mind is to use some Change Management Software to control the modification and logging of source changes, but that may not be an option.
Ideas that spring to mind are:
a) I don’t know if it’s possible or not, but you could investigate adding a trigger to the source files and then when a change is made the trigger program can write an entry to a log file.
b) If you’re using SEU for the changes you could look at putting a wrapper around the STRSEU command that writes an entry to a log file.
c) Train your programmers to always add a “changed by” line in the comments at the top of the source.
All the best