To determine if anyone is stealing your WiFi, check your settings pages for your wireless router. Every device connected to your network has a unique IP address and MAC address, so you can see a list of connected devices. Many devices broadcast an ID because they've been named by their owners (i.e., "John's Laptop") so if you see one connected that you don't recognize, someone is stealing your WiFi. Alternatively, count the number of devices connected and compare that to the number of devices you know there should be to see if they match up.