The IP information on OS X can be found under System Preferences > Network > Wi-Fi > Advanced > TCP/IP.
On Windows you can find the information under Control Panel. In the search box, type adapter, and then, under Network and Sharing Center, click View network connections.
Select your Wi-Fi connection, and then, in the toolbar, click View status of this connection. (You might need to click the chevron Picture of the chevron icon to find this command.)
Your computer's IP address appears in the Value column, next to IPv4 Address.
On iOS it is under Settings > WiFi > [your network (i)].
One way we can attempt to rule your Wi-Fi network in/out is by using an ad-hoc Wi-Fi network. Instructions for doing so can be found here:
Firewall software can also cause problems for us. Temporarily disable your firewall software while connected to an ad-hoc network.
Also make sure you do not have multiple network connections, which can confuse things. If you are connected both via Wi-Fi and an ethernet cable, for example, please disconnect the cable while troubleshooting.
Wi-Fi sync uses Bonjour to announce its availability on the network. If Wi-Fi sync is enabled but they can't see their computer from their phone, and you've done the troubleshooting to rule out their network as the problem, check and make sure this announcement is being made.
On both Mac and Windows this can be verified on the command line with the command
dns-sd -B _services._dns-sd._udp | grep 1p
This should have a single listing that looks something like this:
11:40:39.963 Add 3 4 . _tcp.local. _1password4
(note: Even 1Password 5 for Mac announces as 1Password 4)
On a Mac you can find the name of your computer under System Preferences > Sharing.
On Windows this is found in the Control Panel > System and Security > System.