http://moodle.org/mod/forum/post.php?reply=349457

Quick rundown of the technique I've been using today to work in the guts of
forum cron with confidence...

• Grab a production database, full of users with real emails, and real forum
  posts. Go, mangle those emails a bit just in case.
• Make sure your dev machine doesn't have an MTA. It's safer that way...
• Make sure $CFG->smtphost = '';
• In your SQL utility, say: UPDATE mdl_forum_posts SET mailed=0 WHERE created
  > >= 1183611278 AND mailed!=0; UPDATE mdl_modules SET lastcron=0 WHERE
  name='forum';
• On the commandline, run your cron script setting the sendmail_path: php -d
  sendmail_path=/path/to/modle/admin/mailout-debugger.php

So with this trick, I

• Create a "baseline" run, and mv the log to baseline.log
• Change the code wildly
• Reset the 'mailed' flag and re-run the script
• mv it to wip.log
• Compare the new log with the baseline, skipping lines that have timestamps
  and other variant bits
• Did what you wanted to change change? Other things, did they say the same?

To run the comparison with the proper exceptions, use

diff I '(-oi|boundary|-b|====)' baseline-B2.log wip-B2.log

You can use sdiff with the same commandline to get a nice side-by-side.