To get to the truth, only two people need concern us; Johann Bessler himself and Karl the Landgrave of Hesse who saw the inside of the machine and confirmed that it was genuine. How can we tell if either or both lied?
If Bessler lied then he was taking an almighty risk. He had engaged the attention of several people of high reputation and standing within three Princedoms. In each case he requested and received official examinations of his machine in front of Ministers, Clerks of the Court and religious leaders and of course the ruling Prince or members of his family. Any hint of duplicity and Bessler would face imprisonment and possibly execution as a deterrent to others. He stated in his Apologia Poetica that if he was found to have lied he should be beheaded.
To secure their territory against attackers, usurpers and law breakers, the Princes dispensed justice swiftly and violently. Executions were public spectacles involving cruel methods. In addition, capital punishment was not reserved solely for the most serious crimes. Death was the penalty for a variety of minor offenses. Bessler must have believed that he would be executed if he was found to be lying, so it seems obvious to me that he only told the truth about his machine.
One form of execution popular in Germany was the breaking wheel. It was also known as the Catherine wheel or simply the wheel. 'It was a torture device used for capital punishment from Antiquity into early modern times for public execution by breaking the criminal's bones/bludgeoning him to death. As a form of execution, it was used from "Classical" times into the 18th century; as a form of post mortem punishment of the criminal, the wheel was still in use into 19th-century Germany.' I can imagine someone might find that means of execution highly appropriate! See http://en.wikipedia.org/wiki/Breaking_wheel
But if, for the sake of argument, we assumed he lied, how was he expecting to get his financial reward and leave without being arrested and thrown in jail? He couldn't just run; the deal was that the buyer and the seller sit around a table and the purchaser puts a bag of money on the table and takes the wheel. It seems to me that it would have been impossible for Bessler to leave without the wheel being opened and inspected and verified. This thought must have been considered, before he went ahead with the negotiations.
All the above notwithstanding, if Bessler had lied then either he fooled Karl or Karl lied too. Yet we know from well-documented history that Karl was regarded as an honest man of tremendous integrity. He was in constant touch with the Kings of England, Sweden and Prussia, acting as an honest broker attempting to negotiate peace between these warring nations. He was also known for his patronage of the latest scientific experiments. He supported Dennis Papin in his steam powered experiments for several years and also financed a number of other fields of research. This man was no fool and would have thoroughly scrutinised Bessler's wheel before giving it his approval.
So we know that Karl did not lie either, but let us again suppose that he did, just for arguments's sake. If Karl lied then he must have foreseen that at some point someone would offer to buy the wheel. If the machine was a fake that fact would soon emerge and Karl would be found out, along with his accomplice, Bessler. His reputation would be gone, his status as an honest broker ruined, his family the laughing stock of Europe. It simply does not make sense.
We are left with a paradox; the wheel worked as Bessler claimed, but the laws of physics as they are currently understood say that it is impossible for a wheel to rotate continuously under the influence of gravity alone. Bessler told the truth therefore the laws of physics are wrong on this point at least.