From a historical perspective in America, it all dates back to the Tammany Hall scandal with 'Boss' Tweed in the 1860s.  In that case, William Tweed's huge ring of political corruption and embezzlement of millions of taxpayers' dollars was outed by some lower level city officials who leaked it to the New York Times.  After the Times got a hold of the story and published a series of inflammatory articles along with some now very famous Thomas Nast cartoons, the nation turned against Tweed and his lackies, landing nearly all of them in jail. 

These days, with all of the cable news choices that we have, the press does more watchdogging than ever before. In fact, it's a little confusing to have so many differing opinions flying around.

It seems like every presidential administration has its issue that the press focuses on. Nixon had Watergate, Reagan had Iran-Contra, Clinton had Whitewater, and so on.

You can find controversial issues at local levels too, often involving some kind of corruption on the part of city and county commissioners who use their positions inappropriately.

The Freedom of Information Act grants the press (and any citizen for that matter) access to government materials that might otherwise be concealed. The very fact that the press is there, ready to report any misstep or misfeasance, is often enough to keep government officials in check. Interestingly, reporters face no sanction for printing matters which might be sensitive or even privileged; yet the person who leaked the information to the press may be criminally liable. The example of Scooter Libby illustrates the point. He outed a CIA operative to the Press, and was punished for it; but the reporter who printed the story became something of a hero. So, the very presence and access of the press to government officials and information makes it a powerful watchdog.

Watergate is a great example. Government wrong-doing was exposed by the press. 

The Valerie Plame case is another clear (and complex) example of the press acting as watchdog. Plame's husband, Joseph Wilson, a government employee assigned to fact check some claims about "yellow cake" uranium relating to Iraq's intentions to build weapons of mass destruction, found that there was no basis to these claims. His findings were ignored by the US government which was on the march to war. So Plame's husband posted an Op-Ed in the New York Times explaining his findings and trying to help avoid an armed conflict. 

As apparent retaliation, the White House "outed" Plame, a CIA undercover agent, putting her life at risk in the field. The press carried this story from start to finish, ultimately causing the resignation of Dick Cheney's chief of staff, Scooter Libby. 

The press also acts as a watch dog as it seeks to confirm what are passed as facts. Fact checking is an important service. Also the press also asks very important question in a probing way. For example, when the press demands tax returns, then it puts politicians on the defensive as they are forced to divulge their spending, charitable giving and income. We can see this clearly in our presidential election.

Watergate is, indeed, a great example of the press serving as a watchdog.

Publication of the Pentagon Papers by the New York Times in 1971 is another specific example of the press forcing the government to disclose information that had been hidden from the public for many years.

The Papers revealed that the U.S. had deliberately expanded its war with bombing of Cambodia and Laos, coastal raids on North Vietnam, and Marine Corps attacks, none of which had been reported by media in the US....The most damaging revelations in the papers revealed that four administrations, from Truman to Johnson, had misled the public regarding their intentions.

Perhaps the clearest example of this came when the press did more to uncover the Watergate scandal than anyone else did.  Through such investigative journalism, the press can ensure that wrongdoing by political officials is made public.  This is one way in which the press acts as a watchdog.

It's interesting to note that all major government officials and agencies have their press agents. The press agents act as the liaison between the government and us, the people. While I don't entirely believe everything the press has to report I do believe that the government is slightly intimidated by them, to the point that they will go to great lengths to keep something from them that could injure their standing or position.

We've all heard the saying when something major happens, "The press are going to have a field day with this," meaning they are all over everything that happens everywhere and are going to tell all. Investigative reporters have often uncovered scams and scandals and blown them wide open. In that respect they are sort of "watch dogs" on the government.

The press is always eager for a story to report.  They watch government officials for scandal in their personal lives but they also watch for missteps and poor political decisions.  The media would report quickly if a representative voted in a way contrary to the wishes of his or her constituents.  The media will also report any new proposals or political moves.  Often, the media is a link between the government and the people.  They play an important role in keeping politics relevant to the people and therefore keeping politicians on track.  It is this role as a link between the political world and common man that allows media to be a type of watch dog for our government.