Taken from Capita's SupportNet (KB72234):
If your Named Instance is called 'SIMS' and is installed on the D:\ drive
D:\Program files\Microsoft SQL Server\MSSQL.1\MSSQL\LOG\ERRORLOG.
(Please note the above file has no extension, but can be opened in Notepad.)
Look for a line ending in the following: -
SQL server listening on 127.0.0.1: xxxx
xxxx is the port which your installation of MS SQL Server is attempting to listen on, and this is the port which you need your Firewall to be aware of.
Hope this helps.