Well, if your unspecified hardware supports SNMP, the list of connected MAC addresses is easily available. If not, there are ways of sniffing the LAN for new MAC addresses such as Ethereal.
Search Google for "Wireless Intrusion Detection". There are quite a few listed. If you just want something simple for Windoze, see:
don't have a huge amount of experience with AirSnare, but it does seem to work. My main complaint is that it requires that a monitoring PC be turned on all the time. I prefer sending the collected data to a syslog server, and analyzing the results at my non-existent leisure.
I'm not sure you would want "who tried to connect". When I do that, I get every passing wireless PDA, neighbor, war driver, and such. It also requires that the sniffing be done on the wireless side of the bridge. A connection attempt, with no success, will not show up with any packets on the wired LAN side. Basically, the MAC address sniffers only detect successful connections (those with correct WEP/WPA keys).
There are ways to know, but it might be complicated, expensive, and not necessarily working with all Hardware. Peeking into the Wireless Router? DHCP list usually provides the current IP and MAC of the current users. However you can Not spend your days peeking every few minutes into the Router?s DHCP list.
The common Approach to Wireless (unless you want to provide a free hot spot) is to secure the system and make it so that only computers with permission can log on.
Wireless Security -
WEP, WPA, and the Future -
Or and to isolate your main Network from the Wireless.