Well, there are several ways to go about doing this. One, you can pay a company to perform "penetration testing" of your network. Two, you can frequently convince security companies to do a free penetration test of your network - many will do it for free because they want your business (managed security, etc.). One company that has been advertising free testing is Qualysis (I'm not associated with them, get no kickbacks, etc - just saw their ad. pointing to this link - http://www.sunbelt-software.com/product.cfm?id=545)Three, you can test it yourself, although unless you have a lot of experience in security you won't really know what to test for, and probably won't end up with an accurate picture of the tool's effectiveness. Four, you wait for hackers to test it for you, and keep your fingers crossed that it works...Although realistically, you need to remember that no tool is 100% effective - the best way to maintain security is through constant viligance (applying security patches, analyzing logs, etc.).
Hope this helps!