A document can be falsified, with evidence it is a fake, such as the Lachout document. Or, with evidence to prove the information it contains is false or mistaken, such as the World Almanac on the Jewish population before and after the war.
An eyewitness can be falsified with evidence they did not see what they claimed they saw. Or that they were not in the place they claimed to be. Or with corroborating evidence that proves what they claim bears little to no resemblance to what actually took place. Eyewitness can be partially falsified, as in part of what they claim is evidenced to be true, or correct or a good recollection of events and part is wrong, mistaken or even a lie. Pretty much every single eyewitness to gassings and shootings can be partially falsified, as it is likely they at least made mistakes, when recalling what they saw.
A photo can be falsified, by proving it is manipulated, such as the photo of the body being hung off a tank barrel, or the painfully thin barracks inmates. It could also be shown to not be what it is claimed to be, such as the photo of smoke at TII, which has been claimed to be of corpses being burned, rather than when the rebellion broke out.
Forensic and archaeological evidence can be falsified by evidence its results are wrong. For example, Richard Krege's TII GPR survey and Leuchter's testing of samples from the Krema walls. Often the test results are disputed, as some would argue Green and Marchiewicz are wrong about the chemistry. Falsification can be disputed. One man's accurate test results can be another's total fabrication.
Physical evidence can be falsified by evidence it is not from where it is claimed to have come from, or it was not there at the time claimed, or it has been fabricated and it is fake. For example, in 2011 it was announced that part of a gas mask, a shower head and a vent had been found in the ruins of Krema II, during work to stop it from flooding. If evidence came to light that the objects had been made elsewhere and planted in the ruins, those objects would be falsified.
Circumstantial evidence, by its nature of being evidence from a source type above, can be falsified as described above. For example, the document recording shower fittings inside the Krema can be falsified as other documents can be. That document can be disputed, as in shown to not be evidence of gassings, but instead it is evidence of showering. Circumstantial evidence is normally subject to dispute, a different interpretation, rather than being falsified.
That leads on to how can a historical event be falsified? Sanity Check, gives the following example;
https://codohforum.com/viewtopic.php?p=21455#p21455
Ways to falsify that story would be to check with the venue staff and did they see Mosley speak that night? Could it be the date can be falsified, as in it was in fact November 1933? Or, ask Mosley, and see what he has to say and can he provide the names of others who were there?How do you 'falsify' this newspaper story? Is it a 'scientific theory' that Oswald Mosley spoke at a public meeting in Exeter in December 1933? Or is it just a historical event, an episode in the life of Mosley and the existence of the BUF in public life from 1932-1940? It is certainly documented in this newspaper.
That would be a way the gassing claims about the Kremas could be falsified. Ask the camp staff, who worked at the Kremas, what happened? Look for documents to see if they confirm or not the gassing claims. Examine the buildings, to see if there are any physical signs they were used for gassings.
Claims about historical events cannot be falsified, by arguing they were not possible, when they could have happened. Such claims can be falsified, if they cannot have happened. Or, there is part falsification. For example, a claim that 200,000 people saw the Beatles in the Cavern at their first gig there. That is not possible due to the size of the place, but it does not mean that the claim the Beatles played the Cavern has been falsified, as they, venue staff, photos and recordings prove they played there. It is only the size of the crowd that has been falsified. A claim that the British fired rockets at Berlin in 1945, can be falsified by the lack of any evidence that happened and the lack of technological knowhow. That the Nazis fired rockets at London, is proven by evidence from multiple sources and their technological knowhow. However, a claim that a rocket carried a nuclear warhead, can be falsified due to the lack of knowhow. When it comes to the gas chambers, they were well within Nazi technological capabilities. It cannot be successfully claimed they were not possible. To falsify the gas chambers, would need evidence they did not happen, from people who were there, documents or other evidence.
With regard to the eyewitnesses, to falsify the gas chambers, either find Krema staff who said there were no gas chambers and what they were used for, or prove the eyewitness lied about gassings. Such proof does not come from arguing what they describe is physically impossible, when it was a physical possibility, as shown by the example of the Beatles playing the Cavern. It does not matter if an eyewitness describes a crowd far too large for the venue, or they get the date wrong for the gig. It is physically possible for the Beatles, a band in Liverpool in the 1960s, to have played the main venue in the city and it is well evidenced they played there. Eyewitness can make all sorts of mistakes about the gig, that does not falsify it.