Congress is able to oversee federal bureaucracy by holding the funding that will support the bureaucracy; Congress is also able to initiate and conduct investigations on any bureaucracy that it deems necessary. Congress is responsible for the oversight of each of the federal bureaucracy agencies.
Since Congress was not able to handle and establish everything that was necessary for the growing economy, the federal bureaucracies were created that congress can delegate these tasks to. These bureaucracies are part of the government, but are not officially associated with any single branch of the government. They are, however, directly regulated by congress to ensure that they are upholding their obligations and are staying true to their original missions.
Many of the agencies that are run by Congress must undergo periodic evaluations. This ensures that the agencies are acting the way they are supposed to and are upholding all of the regulations that have been imposed on them. One such agency that does not have to undergo periodic investigations is the Social Security Administration. NASA is an example of an agency that undergoes periodic evaluations. The more often a contract for evaluations with congress, the more control congress has over the way that the agency functions.