first question is why do you think someone is using it?
have you got WiFi security enabled?
which router (make/model)
because you are asking the question - I don't want to overface you
you want a sniffer
first open a command prompt
typr ipconfig {enter}
connection-specific DNS Suffix . :
IP Address. . . . . . . . . : 192.168.254.1 - this is your PC
subnet Mask . . . . . . . : 255.255.255.0 - ignore
default Gateway . . . . : 192.168.254.254 - this is your router
I think the easiest to interpret is
http://www.networkactiv.com/NetworkActivPIAFCT Mv2.2.exe
But - it doesn't work with a wifi adapter - so you'll have to use a wired connection
d/l and install
http://www.networkactiv.com/NetworkActivPIAFCT Mv2.2.exe
click graphical packets mode
press enter
you'll get a real time map of what is talking to what
192 addresses are your internal network - if only you are surfing there will be your address and the gateway address
when you surf ... you'll get addresses that DON't start with 192
an alternative is Look@LAN
http://www.lookatlan.com/oldindex.html
d/l and install
http://www.lookatlan.com/download/LALSetup.exe
create a profile
(if your gateway is 192.168.1.254)
the range will be 92.168.1.1 to 192.168.1.254
it will scan your netwirk at fixed intervals ... and tell you what's connected - and will warn you when another address connects.