How to list extensions in PostgreSQL

Question: I've added an extension on PostgreSQL  during the creation of a DBLink extension for cross database queries. The command I've executed is CREATE EXTENSION dblink.   

How can I list all the extensions within the PostgreSQL db , to confirm the extension is registered?

Answer:    To view information on PostgreSQL extensions you can either use the psql command or you can use some sql code 

>>psql method

\dx

>>sql code method 

select * FROM pg_extension;

>>view available extensions

select * FROM pg_available_extensions