Since there are several definitions of hacker that people use today, I'm going to define it and then answer your question. For the purposes of answering this question, a hacker is to be defined as someone who is not supposed to have access to your system, but for the fun of it has accessed your system. If this individual(s) identifies themselves to you, you might hire that person to help you understand your systems' and network's vulnerability.
Rather than hiring a hacker as defined above, you can hire any number of reputable firms to do a security assessment on your system. They typically perform a "gap analysis" - that is, tell you what the differences are between best practices and what you have implemented. In addition, there are reputable firms that perform "ethical hacking." That is, with your prior knowledge and approval, they will attempt to "break into" your networks and or systems. This can also be called "penetration testing."
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This was first published in August 2002