Winston's work involves falsifying history, yes, but there's no physical proof that's what he's doing.
On paper, his job is not to change the record of historical facts or to falsify evidence of the past: instead, it's to correct 'mistakes' in past news reports etc. Officially, he's employed by the Ministry of Truth (truth, not falsification) to amend the many unaccountable errors in past documents which don't match the present ones. In reality, he knows that he is concealing facts and creating lies to match the current Party line, but there's no solid incontrovertible evidence that this is what he's doing. He could simply believe that he really is correcting mistakes and verifying truths, and no physical evidence in his day-to-day life would contradict this.
Consider, for example, the examples we see of some of his daily orders (Part 1, Chapter 4):
times 17.3.84 bb speech malreported africa rectify
times 19.12.83 forecasts 3 yp 4th quarter 83 misprints verify current issue
times 14.2.84 miniplenty malquoted chocolate rectify
times 3.12.83 reporting bb dayorder doubleplusungood refs unpersons rewrite fullwise upsub antefiling
And the explanation of these weird Newspeak instructions:
it appeared from ‘The Times’ of the seventeenth of March that Big Brother, in his speech of the previous day, had predicted that the South Indian front would remain quiet but that a Eurasian offensive would shortly be launched in North Africa. As it happened, the Eurasian Higher Command had launched its offensive in South India and left North Africa alone. It was therefore necessary to rewrite a paragraph of Big Brother’s speech, in such a way as to make him predict the thing that had actually happened.
‘The Times’ of the nineteenth of December had published the official forecasts of the output of various classes of consumption goods in the fourth quarter of 1983, which was also the sixth quarter of the Ninth Three-Year Plan. Today’s issue contained a statement of the actual output, from which it appeared that the forecasts were in every instance grossly wrong. Winston’s job was to rectify the original figures by making them agree with the later ones.
As short a time ago as February, the Ministry of Plenty had issued a promise (a ‘categorical pledge’ were the official words) that there would be no reduction of the chocolate ration during 1984. Actually, as Winston was aware, the chocolate ration was to be reduced from thirty grammes to twenty at the end of the present week. All that was needed was to substitute for the original promise a warning that it would probably be necessary to reduce the ration at some time in April.
The reporting of Big Brother’s Order for the Day in ‘The Times’ of December 3rd 1983 is extremely unsatisfactory and makes references to non-existent persons. Rewrite it in full and submit your draft to higher authority before filing.
Note the phrasing of the instructions Winston receives. Big Brother's speech wasn't mistaken in view of later events; it was "malreported". Since BB is always right, he must have predicted what actually happened, so the previous report must be wrong. Perhaps agents of Goldstein had been at work falsifying the reports to make it seem as though BB had been mistaken! Ditto with the other instructions: there were "misprints" and things "malquoted", so of course it's the duty of a good citizen to set these mistakes right.
We also see a massive Ministry of Truth operation when Oceania's ally and enemy switch positions (Part 2, Chapter 9):
On the sixth day of Hate Week, after [...] the general hatred of Eurasia had boiled up into such delirium that if the crowd could have got their hands on the 2,000 Eurasian war-criminals who were to be publicly hanged on the last day of the proceedings, they would unquestionably have torn them to pieces — at just this moment it had been announced that Oceania was not after all at war with Eurasia. Oceania was at war with Eastasia. Eurasia was an ally.
There was, of course, no admission that any change had taken place. Merely it became known, with extreme suddenness and everywhere at once, that Eastasia and not Eurasia was the enemy. Winston was taking part in a demonstration in one of the central London squares at the moment when it happened. [...] On a scarlet-draped platform an orator of the Inner Party, a small lean man with disproportionately long arms and a large bald skull over which a few lank locks straggled, was haranguing the crowd. [...] The speech had been proceeding for perhaps twenty minutes when a messenger hurried on to the platform and a scrap of paper was slipped into the speaker’s hand. He unrolled and read it without pausing in his speech. Nothing altered in his voice or manner, or in the content of what he was saying, but suddenly the names were different. Without words said, a wave of understanding rippled through the crowd. Oceania was at war with Eastasia! The next moment there was a tremendous commotion. The banners and posters with which the square was decorated were all wrong! Quite half of them had the wrong faces on them. It was sabotage! The agents of Goldstein had been at work! There was a riotous interlude while posters were ripped from the walls, banners torn to shreds and trampled underfoot. The Spies performed prodigies of activity in clambering over the rooftops and cutting the streamers that fluttered from the chimneys. But within two or three minutes it was all over. The orator, still gripping the neck of the microphone, his shoulders hunched forward, his free hand clawing at the air, had gone straight on with his speech. One minute more, and the feral roars of rage were again bursting from the crowd. The Hate continued exactly as before, except that the target had been changed.
The thing that impressed Winston in looking back was that the speaker had switched from one line to the other actually in midsentence, not only without a pause, but without even breaking the syntax. [...] The instant that the demonstration was over he went straight to the Ministry of Truth, though the time was now nearly twenty-three hours. The entire staff of the Ministry had done likewise. The orders already issuing from the telescreen, recalling them to their posts, were hardly necessary.
Oceania was at war with Eastasia: Oceania had always been at war with Eastasia. A large part of the political literature of five years was now completely obsolete. Reports and records of all kinds, newspapers, books, pamphlets, films, sound-tracks, photographs — all had to be rectified at lightning speed. Although no directive was ever issued, it was known that the chiefs of the Department intended that within one week no reference to the war with Eurasia, or the alliance with Eastasia, should remain in existence anywhere. The work was overwhelming, all the more so because the processes that it involved could not be called by their true names.
Note that last sentence: the processes could not be called by their true names. Nobody ever admits that what they're doing is falsifying historical records. No, no, no: they're correcting historical records to match the 'facts' they know to be true today. There is no single incontrovertible piece of evidence that Winston can point to in order to prove that his job involves falsification. His only ally in knowing what really happened and what didn't is his own memory, which could always be discredited if he chose to speak out.
That scrap of newspaper is the sole exception. He had held in his hands physical, documentary proof that the Party official record was a lie. But now that he's thrown it away, that too exists only in his memory. For just one moment, he might have had the power to expose the Party. (In practice, he would probably have been killed before he could do anything about it. But even so.)