Basically: want to know who is on my WLAN at this moment?

Greetings,
I run a WLAN for the purpose of allowing my B&B Inn's Guest exclusive/limited/convenient access (i.e. no big downloads, etc... - I am using Satellite based internet service: not so great bandwidth). Also, of course, I run my own back office computers on this network. All Linksys stuff, starting with WRT54G router all the way down to Linksys NICs, and via WET54G (wireless bridge) and WRE54G (range extender) and WAP54G and wired switch and hubs.
I use WAP encryption and a custom SSID (I leave SSID broadcast on however) and I diligently change the encryption weekly (or more often sometimes, depending upon how many users I have had on the WLAN). I end up issuing my Guests a little card with the current encryption code on it.
I would like however to see when someone gets on the WLAN - of course, should be either me or Guests - just to make ABSOLUTELY SURE that I do not have hackers onboard and to see how many users I have at a time (never that many anyhow!)
Any way to do this type of basic monitoring?
I was using DELL's NETWORK ASSISTANT for a while - a utility that came with the DELL Inspiron 9400 I bought in 2006: it was for a while letting me know of the computers connecting to the WLAN, but stopped doing so lately. I am not sure why, and other DELL users have complained also of the fickleness of that (really not well supported by DELL) utility, which I want to just drop.
Thanks you for the assistance!
- Roger T

IIRC, I believe this will show you all clients by MAC address:

Go to the router at 192.168.1.1

select the "status" tab

click on the "local network" subtab

click the "DHCP CLients Table" button

Try downloading Look@lan utility at www.lookatlan.com - Network Monitoring and Management Solutions , it's free!!!!!