Findout SQL is listening on which port

USE master
GO
xp_readerrorlog 0, 1, N'Server is listening on'

SQL 2005
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL.\MSSQLServer\
SuperSocketNetLib\TCP\

SQL 2008
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL10.\MSSQLServer\
SuperSocketNetLib\TCP\