The probability of that happening is very low.
If he's worried about people tapping into his connection, most
likely it's open.
If it is encrypted, even with just WEP, you're pretty safe. The
chance that someone with the skill and desire to crack the
encryption, would come along, is low.
If you really care about wireless security. use WPA2 with AES. No
one can crack that in any reasonable amount of time.
It makes more sense to enable encryption than to constantly check
if there are unauthorized users on your network.
Anyway, I suspect the problem is more likely to be with the ISP or
wireless interference. I've been experiencing very slow speeds on
Starhub.
To check for interference, check what's the signal to noise ration
of your wireless connection. If you wireless client doesn't give
you this info, download netstumbler and use that.
