They're accountable to elected officials, but those officials are surely never personally involved in auditing actual behaviour, do they hire people to do that

They're accountable to elected officials, but those officials are surely never personally involved in auditing actual behaviour, do they hire people to do that or do they just rely on heads of security services telling them the truth? (I appreciate that question probably doesn't have a simple answer in any one country, yet alone as a vague thought about security services around the world.)