Certain routers allow you to check if there are devices connected to your network.
My Cisco router for example, comes with a software that lets me see the whole network. I can kick devices out from the network, but that won't be necessary. Because I have MAC address filtering + a complex WPA2 password + disabled SSID broadcast. Which is secure enough.
Like me, you can use some of the router's security features, MAC address filtering, WPA2 encryption (if you have it), disable SSID broadcast and you should be fine.