Oversight of the executive branch is an important Congressional check on the President's power and a balance against his discretion in implementing laws and making regulations.
A major way that Congress conducts oversight is through hearings. The House Committee on Oversight and Government Reform and the Senate Committee on Homeland Security and Government Affairs are both devoted to overseeing and reforming government operations, and each committee conducts oversight in its policy area.
Congress also maintains an investigative organization, the Government Accountability Office (GAO). Founded in 1921 as the General Accounting Office, its original mission was to audit the budgets and financial statements sent to Congress by the Secretary of the Treasury and the Director of the Office of Management and Budget. Today, the GAO audits and generates reports on every aspect of the government, ensuring that taxpayer dollars are spent with the effectiveness and efficiency that the American people deserve.
The executive branch also polices itself: Sixty-four Inspectors General, each responsible for a different agency, regularly audit and report on the agencies to which they are attached.