First of all I don’t think that there is any documented way to access ICS table.
However, if you filter on the internal interface(LAN) you can differentiate packets destined to the external network (Internet) from the packets destined to the ICS server by their destination IP address. It is hardly possible possible on the external interface (after NAT applied), however you still can use information gathered (build your own copy of NAT table) on the internal network card.