There's a label specifically for writing the IP address on the module, but of course it's seldom used. Fortunately it also includes the MAC ID, which is helpful if you need to recover or reconfigure the module.
The default switch setting to 999 means that the module is either set for DHCP, or it's set to use an address stored in flash memory.
If you handed me one of these devices off the shelf, I would plug just it and my PC into an unmanaged switch, with my Ethernet port set for 192.168.1.x, and I would run both RSLinx Classic and the Rockwell BOOTP/DHCP utility.
If the module's MAC ID shows up in a DHCP request, great: I know that the module is really in out-of-the-box conditions and can set it for whatever address I need, then disable DHCP.
If the module replies to the RSLinx EtherNet/IP driver, that also tells me its IP address.
If neither of those work, I would run Wireshark on that port and look for the module to announce itself via ARP packets (as dmroeder points out).
Its this a little harder than it needs to be ? Yes, but not by a lot. Wireshark, the DHCP/BOOTP tool, and RSLinx are the minimum tools anyone using EtherNet/IP should have at hand when setting up an automation system.