You can run:
postgres=# SELECT pg_reload_conf();
.. from within
kill -HUP the
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.