[Wallace]

Sorry for the delayed reply. I was shopping in Tesco's when I got the email!

You can use either the Airport Utility on the PC or the app on the iPhone.

PC:
Start the Airport Utility as normal. In the left hand side (LHS) box, double click on the device you want to 'look at'. In may case I have two. An Extreme and an Express. On the summary screen about half way down you should see a line that says Wireless Clients followed by a number. If the number is 0 (zero) the obviously you are not connected via WiFi to that particular device. However, if you left click on the words 'Wireless Clients', it will show you who is connected, speed, noise levels etc.

iPhone App:
Fire up the app. The front page will give you a pictorial overview of your devices. Again for me it shows both my Extreme and Express.
Tap on one. If you are connected to that device, you will see a line 'Wireless Clients and a number. Tap on the wording again and it will list the MAC address of the connected device/devices. Tap again on the MAC address and it gives connection details and quality etc.

HTH.