One sign to check whether the monthly scripts have been executed is to look at the log file in /var/log/monthly.out. If that file were missing of if it did not show results from regular monthly runs, then here's why:

The default setup is to execute the scripts on the first of every month at 5.30 a.m. To change that on Mac OS X, you will want to modify this file: /System/Library/LaunchDaemons/com.apple.periodic-monthly.plist

[Update: If the plist uses Interval and GracePeriod then don't change it; it is already solved].