@turquoise-- I'm not an expert on this topic but I think you can tell by keeping track of how many megabytes and gigabytes you usually use.
I use a WiFi wireless network. My network main page actually shows how many computers are connected to my internet at that time. I don't know if there is a way to hide a connection, but if there isn't, this is a reliable way to check.
Also, if you suddenly see a huge change in the amount of megabytes and gigabytes of data you download and upload through your WiFi, that's a good sign that someone is using it illegally.
Older wireless networks were at greater risk of WiFi sniffing. Recent ones however have a lot of precautions like coding to avoid this. So you might want to switch to a newer system if you're worried about sniffing.