Identify user requested queries

When the user attempts to make a connection with the database , the code will look into various pg_catalog tables internally. However the user also can query the pg_catalog tables. Is there a way to identify the user requested (or typed query) vs the system requested (internal) queries? 

Also what procedure or function in the code that indicates the user can write queries , something like I wanted to know the code where the connection is created and available for user to use.

Please Help!!

Praveen