Chris Beck wants to find out if there are unauthorized users on his wireless network.
If a stranger successfully gets onto your WiFi network, they may be stealing more than bandwidth. After all, they have access to your computers. Your home network should be more secure than the café's down the street.
The first thing you need to do is make sure that no one can get into that network easily. That means setting up your router to use WPA, or better, WPA-2 security.
Because I don't know what kind of router you have, I can't tell you exactly how to do that. Check your router's documentation. Generally you enter a particular IP address into a browser, which will take you to an HTML-based configuration program located inside the router's firmware.
Setting up security will require you to create a password. Make it a strong password that no one will be able to guess. But remember that you'll have to enter that password into every computer, smartphone, Blu-ray player, or other WiFi-equipped device in your home, including overnight guests' laptops. HDTVs and Blu-ray players are the worst for entering passwords - remote controls just aren't friendly for text entry.
Okay, I've told you how to control who gets access, but I didn't actually answer your question: How do you see if someone is on your network who shouldn't be?
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.