How to determine which one is my default instance in MS SQL Server? I have two instances in my server and I want to know which one of them is the default. Can you, please, point out ways I can recognize it. 
By saying a default instance I mean the one that listens for 1433.

Thanks for your support.
login in to your server and run this

 SELECT SERVERPROPERTY('InstanceName')


for sql server 2008 and above if it returns null then it's a default instance else named one


  InstanceName: Name of the instance to which the user is connected.
  
  Returns NULL if the instance name is the default instance, if the
  input is not valid, or error.
  
  NULL = Input is not valid, an error, or not applicable.
  
  Base data type: nvarchar(128)


Ref

Another one using SEVERPROPERTY(),

Run the below code

SELECT SERVERPROPERTY('ServerName')


SELECT SERVERPROPERTY('MachineName')


if both returns same value ie machine then it's your default instance else
the first one return with your machinename\instance name

After your edit

if you want to check the SQL server service which listens to port 1433. Then you have to check that in SQL server configuration manager. Check-in SQL server network configuration in that protocols for SQL server and click properties for TCP/IP and see the port used under IP addresses