This question is akin to “How do you know the FBI investigates crime?”, or “How do you know the NSA records and analyzes signals intelligence?” It is the charter of the organization and the basic description of the duties of pretty much everyone who says they worked there. Even the whistle-blowers in the CIA or MI6 explain the criminal or unethical activity, torture, etc., as something done in the execution of or in addition to their main duties as intelligence officers.
This question is akin to “How do you know the FBI investigates crime?”, or “How do you know the NSA records and analyzes signals intelligence?” It is the charter of the organization and the basic description of the duties of pretty much everyone who says they worked there. Even the whistle-blowers in the CIA or MI6 explain the criminal or unethical activity, torture, etc., as something done in the execution of or in addition to their main duties as intelligence officers.