I deployed a Cisco Access Point and did not know the IP address of it.
I've been on onsite many times and the equipment IP in the documentation or label was incorrect?
If I was physically near the unit, my first option would be to connect it to my laptop, start a packet capture and power the unit on. Most IP hosts, will transmit a Gratuitous ARP for duplicate IP address detection, revealing its IP address.
Since I am not near it, I will telnet into the Cisco switch that is connected to the AP, and leverage the show CDP command to determine its IP address.
Enjoy