Find the SQL Server Port Number

Check the SQL error log by using the below TSQL:
sp_readerrorlog 1, 1, 'listening', 'server'


Here is another method for default instance:
DECLARE @findport_number nvarchar(5)
EXEC xp_regread
@rootkey    =    'HKEY_LOCAL_MACHINE',
@key        =   'SOFTWARE\MICROSOFT\MSSQLSERVER\MSSQLSERVER\SUPERSOCKETNETLIB\TCP',
@value_name    =    'TcpPort',
@value        =    @findport_number OUTPUT
print 'The server port number = '+@findport_number