Once again, you turn towards your router's configuration program. Somewhere in there, probably in a menu called Wireless or Status, you'll find a list of all current wireless clients. You won't be able to identify those clients at a glance, but you will be able to see how many there are. That can tell you if there's an extra.

But before you count too many and panic, think hard about what devices may be legitimately using your WiFi. When researching this article, my router told me I had five wireless clients. I could only identify four. Turns out my daughter's iPhone had WiFi turned on.

You can identify the clients you know via their MAC Address, which has nothing to do with Apple Computers (although Macs do have MAC addresses). These numbers are unique for each device that can get onto a network. You can go to each legitimate WiFi client and find its MAC address, although how to do it depends on the device. For instance, on a Windows PC, go to a Command Prompt window, type ipconfig /all, and press ENTER. The MAC address is listed as the "physical address."

If you're wondering about past freeloaders, your router's configuration program should have a log somewhere. It should also have a tool for blocking undesirable MAC addresses.

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