A hoax media release purporting to be from ANZ Bank is reported to have caused a drop in Whitehaven Coal shares. One way to detect a hoax media release is to check the website of the organizations. Companies and government agencies normally have a specific part of their website where they list media releases. Companies on the stock exchange may also have a location for important announcements to the market.
There are more technical ways to authenticate documents with "digital signatures", but simply checking web site will do in most cases.
In 1995 when setting up the first web site for the Australian Department of Defence, media releases were included.
ps: You can check up on me in the ANU website. ;-)