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