Who are the thought police in 1984?
The Thought Police in 1984 are an organ of the totalitarian state charged with ensuring that people don't step out of line. A secret police force, the Thought Police exist to root out all forms of subversion, including thoughtcrimes, which are the most serious crimes of all. Thoughtcrimes are those thoughts that in any way challenge the rule of the Party, which is why they're regarded by the state as so dangerous.
The Thought Police are Oceania's equivalent of the Gestapo or the KGB. They're a secret police force designed to keep the people in check, to stop them from engaging in any behavior that could possibly be deemed a danger to the state.
As one can imagine in this totalitarian society, the powers of the Thought Police are virtually unlimited. The citizens of Oceania have no civil liberties whatsoever, and so the Thought Police, as with all other organs of the state, have no legal restrictions on their operations. They thus have complete freedom to spy on citizens for any reason and for any length of time, and to use whatever tools they deem necessary for investigating subversion and anti-state activity.
The main focus of the Thought Police is in the detection, investigation, and punishment of thoughtcrimes. Thoughtcrimes are those unspoken thoughts and beliefs that are deemed dangerous and subversive by the state.
Although the Thought Police cannot read people's minds, they can still detect thoughtcrimes by closely monitoring their body language through highly intrusive telescreens, which everyone except the proles are required to have in their homes. As this would suggest, there is no privacy in Oceania. Indeed, for the authorities privacy is a dangerous concept as it allows people to get away with thoughtcrimes, the most serious crimes of all in this totalitarian state.