The reliability of an intelligence source, whether in a foreign intelligence or domestic law enforcement context, is a product of the nature of that source. When discussing the reliability of sources of information, one is generally focused solely on human intelligence, or HUMINT. As such, each individual source has to be carefully scrutinized or vetted for reliability. The most reliable human sources of intelligence involve U.S. or allied intelligence agency operatives who have successfully infiltrated a foreign intelligence agency, terrorist organization, or criminal group. Such agents are trained operatives who are are considered extremely reliable. From there, however, concerns about reliability grow immeasurably.
As it is extremely difficult and dangerous to infiltrate a law enforcement officer or intelligence operative into a criminal or terrorist organization, it is more common to glean human intelligence from sources within those organizations. Depending upon the motivations of the individual source, he or she can be considered highly reliable, completely unreliable, or someplace in between, with the latter category being most common. Anytime an intelligence or law enforcement official succeeds in recruiting a source inside a criminal or terrorist organization, the possibility exists that the source is either lying to enhance his or her image in the eyes of the government agency involved, or is deliberately feeding false information to the government personnel because he or she has not actually switched sides.
It is because of the ever-present danger that recruited sources are only pretending to have switched loyalties, any information they provide has to be treated with caution. Intelligence derived from dubuious sources has to be carefully vetted for the possibility that is disinformation intended to lead government agencies astray. In addition, there is always the danger that a foreign agent or terrorist who defects to the United States, and who divulges genuinely valuable information, will change his mind and return from whence he came. Such was the case of KGB agent Vitaly Yurchenko, whose 1985 defection to the U.S. resulted in a major public relations coup for the CIA, but whose subsequent redefection back to the Soviet Union embarrassed those same CIA officials. In between, however, Yurchenko was said to have provided valuable intelligence on the KGB's operations in the U.S. Subsequent to his redefection, however, all of that information had to be reevaluated for possible disinformation designed to lead the CIA away from directions the KGB didn't want it to go.
Most recently, sources inside terrorist organizations have endured enhanced scrutiny by U.S. intelligence handlers after a Jordianian doctor, Humam Khalil Abu-Mulal al-Balawi, convinced U.S. intelligence personnel that he was a defector from al Qaeda, only to detonate a suicide bomb once inside a sensitive CIA base camp in Afghanistan, killing a number of CIA operatives in the process.
In conclusion, a reliable source is one whose motivations and allegiances are well-documented, and whose information is verified for accuracy. Unreliable sources are those whose motivations are questionable, and whose allegiances prove less concrete than previously thought, and whose information proves inaccurate on a recurring basis.