As always, RTFM. Googling finds the manual, and on p. 2-20 it says:
The console can be accessed through these methods:
- Out-of-band: Connect a PC or VT-100 terminal, to be used as a console,
directly to the switch using the serial cable that comes with the Series
5400zl Switches. If the PC or terminal has a 25-pin serial connector, you
can use a readily available 9-pin to 25-pin serial cable, or attach a 9-to-25
pin straight-through adapter to the PC end of the cable.
- In-Band: Access the console using telnet from a PC or UNIX station on
the network, and a VT-100 terminal emulator. This method requires that
you first configure the switch with an IP address and subnet mask by using
either out-of-band console access or through DHCP/Bootp.
So, use the serial cable and configure the IP address, or connect the switch to a LAN segment with a working DHCP server, then the DHCP server will assign it a (transient) IP address (identified by the MAC address of the switch's internal interface).
tcpdump will tell you nothing (an initial DHCP request broadcast if you are lucky) unless the DHCP server is already working, in which case you can just look up the address in the DHCP server data without bothering with
So, there is no "command to tell you the IP address". You must use the correct infrastructure.
If the switch already has an IP address (by console or DHCP), and if you are not able to sniff the DHCP negotiation (because you can't reboot the switch, and/or the negotiation happens on a different segment), then you are just out of luck.
You can try to scan your IP range (you have got an IP range from the DHCP server behind the switch, haven't you?) with
ping etc. and hope you get an answer, but if the switch is configured to have its internal IF only on the segment where it got the DHCP answer, you are again out of luck.
Best solution is to have someone go there once, access the switch physically, fix the IP address in the console, and tell everyone who needs to know.
If you don't have permission to access the switch physically, you probably also don't have permission to access its console interface.