Wireless Network Watcher is a free, simple program for Windows XP and above that can take a quick look at the computers and devices connected to your home network. Fire it up and make sure there's no one on your Wi-Fi network who shouldn't be there. If you do find a squatter, a password change on your router should be enough to thwart them.
Once you've downloaded and installed the application from the Nirsoft site, you're ready to goâ€”there are no setup screens to wade through or configure. Launch the program executable to see all of the devices currently connected up to your router, which will itself appear at the top of the list.
Some detective work may be required to work out what's what. The IP and MAC addresses are the identifiers used by your network hardware to recognize each device, but they won't be much use to you right off the bat. Of more interest are the Device Name, Device Information, and User Text fields that should give you some idea about which laptops, tablets and mobile phones are connected.