You can go to your routers configuration page and check the dchp table to see what clients your wireless is currently giving ips out to. This would be able to tell you how many different computers are connected and if you have 2 computers, yet you see like 3 or 4 different computer names that aren't yours then maybe somebody else is connected to your network. Try this and if it looks like you have someone piggy backing your network you might want to try some new encryption methods or something, as far as the law goes if he is using your connection without permission idk what kinda trouble he would get in but you could research.