Let me try to answer your question in a way that does not spoil what happens in the rest of this excellent novel that should be read by everyone. Well, the first two chapters make it clear, as I am sure you have picked up, that Winston Smith is living in a society where it is very hard to escape the constant surveillance of the authorities. Telescreens abound and helicopters look into people's appartments. The most feared of course are the Thought Police, and although they cannot actualy hear what you are thinking, they have an almost mythic status that is used to instil fear into the population and suggest that they actually can hear what you are thinking. Note what Winston says about them after writing "Down with Big Brother" in his Diary:
Whether he went on with teh diary, or whether he did not go on with it, made no difference. The Thought Police would get him just the same. He had committed--would still have committed, even if he had never set pen to paper--the essential crime that contained all others in itself. Thoughtcrime, they called it. Thoughtcrime was not a thing that could be concealed for ever. You might dodge successfully for a while, even for years, but soner or later they were bound to get you.
Thus there is obviously something that you do, when you commit Thoughtcrime--some change in your appearance or how you do what you do--that makes the Thought Police suspicious about you and lead to your vapourisation. It is not that they have the ability to read thoughts, but that they are constantly watching your every move, and so any suspicious activity can lead to your arrest.