Step 1: Log into your router
To find out what devices are currently soaking up that sweet, sweet speedy signal, you'll need to log into your router. Type the device's gateway IP address into your browser. The two most popular router brands, Cisco and Linksys, share a default address of 192.168.1.1 -- otherwise, you should be able to find yours on this page.
You'll be prompted to enter the username and password, and unless you switched things up when you first installed the router, your username is most likely still the default "admin" and your password is either "password," the router's serial number (which you'll find on the physical router itself), or blank.
Next, click on the tab that reads "DHCP Client List" or "Attached Devices," which will show you a list of the MAC addresses for devices currently using your router's connection. If there're more numbers/devices listed than you know should be on there, you've probably got a squatter.
It may sound a little complex, but it's actually quite simple. However, if the above seems way over your head, NETGEAR, Linksys, and Apple's AirPort each have dedicated apps that make it much easier for tech-phobic folks to keep tabs on who's using their connection.