How to check the syntax of pg_hba.conf and other postgresql conf files on ubuntu/debian/linux?
You can run:

postgres=# SELECT pg_reload_conf();
 pg_reload_conf
----------------
 t
(1 row)

postgres=# 


.. from within psql. or kill -HUP the postmaster process.

Any config validation errors will then get put into the Postgres log file, and it won't reload the config. Example log:

2016-10-05 10:31:57 BST LOG:  received SIGHUP, reloading configuration files
2016-10-05 10:31:57 BST LOG:  invalid connection type "awdawdawdawd"
2016-10-05 10:31:57 BST CONTEXT:  line 1 of configuration file "/etc/postgresql/9.3/main/pg_hba.conf"
2016-10-05 10:31:57 BST WARNING:  pg_hba.conf not reloaded


I'm not aware of a tool that just checks config for validity, but there was a thread on the pgsql-hackers mailing list here that discussed a proposal for a validator.