AWS RDS Postgres pg_hba.conf auth-method
From what I can gather, SSL is usually being used.  What varies is the level to which server certificates are verified.

Reading the guide from Amazon is helpful albeit lengthy.

Things to remember...


To determine if SSL is on (apparently this requires a plugin but works by default on AWS RDS):


  show ssl;

There is a ~/.pgpass file that automatically supplies passwords when using psql (this confused me for a while)
Full server certificate verification (requires the root cert from AWS):


  $ psql -h testpg.cdhmuqifdpib.us-east-1.rds.amazonaws.com -p 5432 "dbname=testpg user=testuser sslrootcert=rds-ca-2015-root.pem sslmode=verify-full"

SSL is still on with this command however (no sslmode,sslrootcert):


  $ psql -h testpg.cdhmuqifdpib.us-east-1.rds.amazonaws.com -p 5432 "dbname=testpg user=testuser"

This will not connect because root cert is not supplied:


  $ psql -h testpg.cdhmuqifdpib.us-east-1.rds.amazonaws.com -p 5432 "dbname=testpg user=testuser sslmode=verify-full"

I never established a non-SSL connection however it is possible to prevent non-SSL connections to a DB instance with a parameter on the server: rds.force_ssl