In the novel 1984, Winston Smith writes:
Thoughtcrime does not entail death: thoughtcrime IS death.
How exactly does Smith, or anyone else, think that Big Brother will discover their thoughtcrimes? Do they think Big Brother can read their minds?
In the very first chapter, in the opening paragraphs of the story, the first time we're introduced to the telescreens, we're also introduced to the normality of permanent all-encompassing surveillance:
The telescreen received and transmitted simultaneously. Any sound that Winston made, above the level of a very low whisper, would be picked up by it, moreover, so long as he remained within the field of vision which the metal plaque commanded, he could be seen as well as heard. There was of course no way of knowing whether you were being watched at any given moment. How often, or on what system, the Thought Police plugged in on any individual wire was guesswork. It was even conceivable that they watched everybody all the time. But at any rate they could plug in your wire whenever they wanted to. You had to live--did live, from habit that became instinct--in the assumption that every sound you made was overheard, and, except in darkness, every movement scrutinized.
My interpretation has always been that the possibility mentioned only as "conceivable" was indeed the truth: that the Thought Police were watching everybody all the time. In any case, people know that it's a possibility, and even the possibility is enough to make people afraid all the time, knowing that everything they do is (or might be) being watched.
Thoughtcrime is not visible, you might say. They can watch everyone, but they don't have mind-reading technology. To that I reply, a good psychologist can figure out what someone is thinking just by watching them closely enough: their body language, their posture and position, their eyes, the pattern of their breathing - all of these things are clues towards someone's inner self. Presumably the Party would have such experts in their Thought Police, people who spend years watching citizens and figuring out what they're thinking. It's in this way that people can be found and "vaporised" before they even have a chance to act on their rebellious thoughts.
Later on, after Winston is captured and tortured, it's confirmed that the Party has been watching him for years, has seen everything he's been doing and deduced his inner thoughts:
He knew now that for seven years the Thought Police had watched him like a beetle under a magnifying glass. There was no physical act, no word spoken aloud, that they had not noticed, no train of thought that they had not been able to infer. Even the speck of whitish dust on the cover of his diary they had carefully replaced. They had played sound-tracks to him, shown him photographs. Some of them were photographs of Julia and himself. Yes, even...
Thoughtcrime is easy to detect when you're watching everything that everyone does all the time. You just need to figure out the reasons behind the acts, and that's easy to do with so much information about the person.
Winston, in the novel, did not die. So to some extent, to say 'thoughtcrime IS death' is not quite true. It's true at the level of how the human spirit can be crushed. Nevertheless, the two totalitarian regimes we do know of in history were defeated. And this cannot be the case if the human spirit remains crushed, and is always crushed under a totalitarian regime. The human spirit is such that it always finds a way, even in the most terrible and adverse of conditions. Remember, the Stalinist system was broken down by the Russian people themselves - it didn't take an external force like it did with the Nazis.
What Orwell is painting is a picture of how a totalitarian system works. He didn't spell out the details, the brushstrokes he painted should be enough. We read peoples minds all the time, surely you've been in situations where you could finish the sentence someone is about to say? It happens all the time. Orwell is relying on an understanding of psychology by the reader, which he can expect, since after all, the novel has one of its important themes - human psychology.
What Orwell was interested in is how a totalitarian regime could co-opt something natural and turn it into something terrible. He was interested in laying bare the twisted logic of it all - from the panoptic Big Brother, the Ministry of Lies, the two minute Hate and so on.
But to concentrate on details is to miss the big picture. After all, Hannah Arendt wrote that the totalitarian regime, although defeated, remains a possibility that the West could take again. And so to avert that possibility requires vigilance. But if it arrives, the details will be different. It's not going to announce itself but hide until its omnipresent.