For the smaller organization there are several layers of control that can be built in to reduce the rogue wireless threat. The first place to start is with policy. All employees should know the rules regarding wireless and what can and cannot be plugged into the network.
Policy enforcement will be easier if you have managed switches. You can disable unused ports and start restricting down active ones by MAC address filtering.
Next, find some tools that will let you scan for rogue access points. There are commercial tools that will do this such as AirMagnet and AirDefense, and if your budget is tight you might want to try an open source tools such as RogueScanner.
Finally, don't be shy about using tools like NetStumbler and other site survey tools to identify access points and verify their legitimacy.
This question was also answered by our wireless networking expert, Lisa Phifer. Read her response to this question: Monitoring your network to detect rogue access points.
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