To track changes you need a source of truth that may be used to compare the changes. Generally it can be a source control location.
The easy way is to connect your higher environment to the Source of Truth SVN location and create a cron job or task schedular (Windows) that will do a SVN compare, and look for changed. Once changes are detected send email.
This way you can make sure that not only code is not changed in one server, it is also in synch across cluster.
Or else you may use tools like Cross Vista. It has similar feature out of the box.
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