Who watches the watchers?

I fully accept that there is a need - in very limited and extreme circumstances - for interrogations of kidnappers and terrorists to be pretty extreme themselves whenever there is 'a ticking bomb scenario'.

I have just been watching 'The Dark Art of Interrogation." Indeed, I had few problems with the methods used in the circumstances described BUT then again, Quis custodiet ipsos custodes?

Who makes these decisions which fall outside the realms of the standard justice system? What are their qualifications? What are their biases? How do we know that only the guilty are being targeted? How do we know that these things will not be used on mere political opponents? CAN WE TRUST OUR GOVERNMENTS SUFFICIENTLY?