How to connect to localhost with postgres_fdw?

After many attempts probably I found a proper way to connect:

CREATE SERVER app_db 
FOREIGN DATA WRAPPER postgres_fdw 
OPTIONS (dbname 'test', port '5432', host 'localhost');

Then:

CREATE USER MAPPING for postgres
SERVER app_db 
OPTIONS (user 'postgres', password 'postgres');

And then:

CREATE FOREIGN TABLE groups
(
  id serial NOT NULL,
  name character varying(255) NOT NULL,
  version integer DEFAULT 0
)
 SERVER app_db OPTIONS (schema_name 'public', table_name 'groups')

But is there a solution to check if it's really "remote" connection? Because servers are on the same localhost and I don't know if I can be sure.

You may connect using Unix Domain Socket instead of TCP connection to gain simpler configuration and better performance (Linux/Unix only, not supported in Windows).

CREATE SERVER app_db
FOREIGN DATA WRAPPER postgres_fdw
OPTIONS (dbname 'test');

Also, you may omit the password if peer authentication is enabled (default).

CREATE USER MAPPING for postgres
SERVER app_db 
OPTIONS (user 'postgres');

Note: peer authentication can only be used for postgres user as the FDW connection is created by the server backend which runs as the system user postgres.

Also, for security reasons, postgresq_fdw allows peer authentication only for clients with SUPERUSER privilege. To allow restricted users to use the FDW, you have to use password authentication as described in this answer