How to get the timezone from a PostgreSQL timestamp
The documentation on Postgres timestamps says:


  For timestamp with time zone, the internally stored value is always in
  UTC (Universal Coordinated Time, traditionally known as Greenwich Mean
  Time, GMT). An input value that has an explicit time zone specified is
  converted to UTC using the appropriate offset for that time zone. If
  no time zone is stated in the input string, then it is assumed to be
  in the time zone indicated by the system's TimeZone parameter, and is
  converted to UTC using the offset for the timezone zone.


Contrary to what a reasonable person might expect when reading timestamp with time zone, this Postgres datatype doesn't actually store a timezone. Internally, the value is always converted to UTC. 

The information about the input timezone is lost when saving the value to the database. As far as I know, there is no way to get the information you seek.