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There are companies or security professionals that can run penetration tests from outside of your network to audit your Firewalls and Network for any vulnerabilities. These companies or individuals should be certified ethical hackers. To learn more about penetration tests click here.

If you are familiar with the technology and the general concept of "hacking" you can usually test your network without any special software however, there is a linux distro called BackTrack that is distributed as a Live CD which resulted from the merger of WHAX (previously Whoppix) and the Auditor Security Collection, which is used for penetration testing. You can learn more about BackTrack here: http://en.wikipedia.org/wiki/BackTrack