PostgreSQL how to see which queries have run

Turn on the server log:

log_statement = all

This will log every call to the database server.

I would not use log_statement = all on a production server. Produces huge log files.
The manual about logging-parameters:

log_statement (enum)

Controls which SQL statements are logged. Valid values are none (off), ddl, mod, and all (all statements). [...]

Resetting the log_statement parameter requires a server reload (SIGHUP). A restart is not necessary. Read the manual on how to set parameters.

Don't confuse the server log with pgAdmin's log. Two different things!

You can also look at the server log files in pgAdmin, if you have access to the files (may not be the case with a remote server) and set it up correctly. In pgadmin III, have a look at: Tools -> Server status. That option was removed in pgadmin4.

I prefer to read the server log files with vim (or any editor / reader of your choice).

PostgreSql is very advanced when related to logging techniques

Logs are stored in Installationfolder/data/pg_log folder. While log settings are placed in postgresql.conf file.

Log format is usually set as stderr. But CSV log format is recommended. In order to enable CSV format change in

log_destination = 'stderr,csvlog'   
logging_collector = on

In order to log all queries, very usefull for new installations, set min. execution time for a query

log_min_duration_statement = 0

In order to view active Queries on your database, use

SELECT * FROM pg_stat_activity

To log specific queries set query type

log_statement = 'all'           # none, ddl, mod, all

For more information on Logging queries see PostgreSql Log.

I found the log file at /usr/local/var/log/postgres.log on a mac installation from brew.