The other way is finding out what has been written for that purpose already.
monit looks like an option:
You may use monit to monitor files, directories and devices for changes, such as timestamps changes, checksum changes or size changes.
I thought cacti or nagios would have support for directory or file size checksums but I could not find that, yet ...
Maybe have a look for yourself.
Other options might be:
- pikt: http://pikt.org/pikt/intro/intro_system_security.html
Although it looks like it can only handle files instead of directories.
- zabbix: http://www.zabbix.com/
Looks a littlebit too big for this simple purpose.