METHOD 1:
The easiest way to determine the IP address of your Foobar2000 server PC is to read it off the File>Preferences>Tools>Control Server screen in Foobar2000.
Make sure your server PC is connected to your local network before you bring up the Control Server preferences screen.
METHOD 2:
In Foobar2000 Copilot on your mobile phone, you can also use the 'scan for foobar server ip' feature listed under the page menu for the connections screen -- if for some reason you aren't near your server PC to use Method 1.
This option will essentially auto scan a range of IP addresses, seeing which one connects to your server.
To use this option, you must have your phone connected via Wi-Fi to your local network where your Foobar2000 server resides.
Foobar2000 must also be up and running and with the 'foo_controlserver' plug-in installed and configured (see : foo_controlserver plug-in install for more details on that plug-in).
Also if you have multiple Foobar2000 servers on the same network, turn off all servers but the one you want to connect to for the duration of the scan, since the scan will stop after finding the first Foobar2000 server it can connect to. But if you have to do this, using METHOD 1 above is a lot easier.
On startup, this feature will detect the IP address of your phone and then use its base IP address for constructing the range of IPs to scan. So for example, if your phone's IP is 192.168.0.10, it will scan from 192.168.0.1 to 192.168.0.255 by default.
Scanning thru 255 IPs can take about 3 minutes, but it is unlikely you will have to scan them all.
The base IP address and range displayed on the screen are editable fields, so you can alter them should you want to limit the range of the scan or should you need to use a different base IP address than the default for some reason.
If the scan finds a Foobar server, it will display its address in the 'Server IP' box. You can select and copy that text should want to next paste it into the 'connections' edit form.