The best answer is to contact your network administrator. Someone should be managing your IP space to ensure there are no conflicts. It's not uncommon for people to ping random IP addresses to find an address that isn't responding, and then assign their system that static address. However, this is dangerous -- you could assign yourself the address of a system that happened to be rebooting. Or, an administrator could reassign that address later, thinking that it was unused. Ultimately, the only way to identify a static IP address without risking other systems on your network is to ask the person who manages your IP addresses.
This was first published in November 2002