How do I determine if pgAgent is installed on a Postgres instance?
From Database setup:


  pgAgent stores its configuration in the ‘postgres’ database in your
  cluster


And it creates there a schema named pgagent with a handful of tables prefixed by pga_

It's probably good enough to connect to the postgres database and interpret the result of:

select 1  from information_schema.schemata  where schema_name='pgagent';


Due to how information_schema.schemata is restricted, the above should be run as postgres user (otherwise consider querying directly pg_namespace as the non-standard alternative).