The easiest way is by using free utility called Wireless Network Watcher v1.55 .
1) Go to nirsoft.net and download Wireless Network Watcher v1.55.
2) Install it on your pc and start scanning. It scans your wireless network and displays the list of all computers and devices that are currently connected to your network.
For every computer or device that is connected to your network, the following information is displayed: IP address, MAC address, the company that manufactured the network card, and optionally the computer name.
That's all :)