Show Active Connections to "Internet Sharing"
You can try arp on the command-line:

NAME
arp -- address resolution display and control
DESCRIPTION
The arp utility displays and modifies the Internet-to-Ethernet address translation tables used by the address resolution protocol (arp(4)).  With no flags, the program displays the current ARP entry for hostname.  The host may be specified by name or by number, using Internet dot notation.

E.g. for internet-sharing from Ethernet to Airport I use:
arp -i en1 -a
This will list all clients connected via WLAN.