How to find the version of postgres installed on a server

$ su - postgres
Last login: Tue Dec 18 23:03:07 CET 2018
$ pg_config --version
PostgreSQL 11.1

Using psql:

[postgres@til0drift-dbteam-sandbox-pgsql01 ~]$ psql
psql (11.2)
Type "help" for help.

postgres=# select version();
                                                 version
---------------------------------------------------------------------------------------------------------
 PostgreSQL 11.2 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-36), 64-bit
(1 row)