By doing their job. Much of journalism is research.
A great example was the Rather-gate memos from the 2004 election, a supposedly "leaked" memo which got published with great fanfare despite the fact that the forgers did make some obvious errors (ie producing a 1970s document in Microsoft Word using default fonts, spacing and parameters). But if the forgers hadn't been complete dumbasses then how would we have ever known whether they were real?
Heck, has a single Wikileaks document ever been proven to be genuine?