If you are connected (using the instance name) you can also find it by query (SQl2000 and up):

set nocount on

go

DECLARE @SqlPort Nvarchar(10), @instance_name Nvarchar(30), @reg_key Nvarchar(500), @value_name Nvarchar(20)

if left(CAST(SERVERPROPERTY('ProductVersion')AS sysname),2) = '10'

BEGIN

select @instance_name = CAST(SERVERPROPERTY('instancename')AS sysname)

if @instance_name is NULL

BEGIN

set @reg_key = 'SOFTWARE\Microsoft\MSSQLServer\MSSQlServer\SuperSocketNetLib\Tcp'

END

ELSE BEGIN

set @reg_key = 'SOFTWARE\Microsoft\Microsoft SQL Server\' + @instance_name + '\MSSQLServer\SuperSocketNetLib\Tcp'

END

EXEC master..xp_regread @rootkey='HKEY_LOCAL_MACHINE',

@key=@reg_key, @value_name='TcpPort',

@value=@SqlPort output

select CAST(SERVERPROPERTY('ServerName')AS sysname) as ServerName, @SqlPort as Port

END

if left(CAST(SERVERPROPERTY('ProductVersion')AS sysname),1) = '9'

BEGIN

select @instance_name = CAST(SERVERPROPERTY('instancename')AS sysname)

if @instance_name is NULL

BEGIN

set @reg_key = 'SOFTWARE\Microsoft\MSSQLServer\MSSQlServer\SuperSocketNetLib\Tcp'

END

ELSE BEGIN

set @reg_key = 'SOFTWARE\Microsoft\Microsoft SQL Server\' + @instance_name + '\MSSQLServer\SuperSocketNetLib\Tcp'

END

EXEC master..xp_regread @rootkey='HKEY_LOCAL_MACHINE',

@key=@reg_key, @value_name='TcpPort',

@value=@SqlPort output

select CAST(SERVERPROPERTY('ServerName')AS sysname) as ServerName, @SqlPort as Port

END

if left(CAST(SERVERPROPERTY('ProductVersion')AS sysname),1) = '8'

BEGIN

Create table #Port_2000 (value nvarchar(20),Data nVarchar(10))

insert into #Port_2000 exec master..xp_instance_regread 'HKEY_LOCAL_MACHINE', 'SOFTWARE\Microsoft\MSSQLServer\MSSQLServer\Supersocketnetlib\tcp', 'tcpPort'

select @SqlPort = Data from #Port_2000

select CAST(SERVERPROPERTY('ServerName')AS sysname) as ServerName, @SqlPort as Port

drop table #Port_2000

END