Yes it is a number of ways are avaliable from network monitor programs or to just have a look at what MAC addreses have been stored on a router, and if you don't recognise them, then block them.
But I wouldn't let that concern you, the simpliest ways to protect yourself from most things, is to see what firewalls your ISP provides, firewall on the router itself, get some firewall stuff on your PC and to enable encryption on your router, even something simple like ASCII, don't worry there will be a guide with your router on how to enable this, very easy, and if your really paranoid, you could even enable a few other options and only allow the MAC addresses of the PC's you want to use...
Lists are endless... But just be careful if you are intending to move sensitive data, then take extra steps, its no hard ship to plug a laptop into a hard wired position for a while...