There no way to search an defined object the NAT-IP by the Checkpoint GUI.

A easy way is:

$ #LOGIN AS root ON FW1 via SSH/TELNET
$ grep 181.76.6.7 /etc/fw/conf/objects_5_0.C

Another way is to define a new object with the NAT-IP. The GUI will warn you, that the object is already in use by Object “xy..”.