How people can be sure that somebody, cooperating with central bank (or working in it), is not creating money in digital or paper form for themself, so that nobody else notices it?
In modern economies, “money” is a liability of some entity. If it creates “money,” it is adding to its own liabilities. (E.g., bank deposits are liabilities of banks, and so if they just increased their deposits without offsetting lending activity, they just lowered their net worth.)
The only “cheating” is counterfeiting: issuing instruments that pretend to be liabilities of some other entity. Most large retailers have systems to detect counterfeit currency, so this is already a known factor. As for electronic money, security against fraudulent transactions (similar in effect to counterfeiting) is also an ongoing concern.
What about the government? There are many people who question the truthfulness of government statistics. However, the developed countries publish comprehensive statistical databases, and it is possible to cross-check the data against the other sources, as well as private data. (As an example, Chinese GDP statistics were often compared to electricity generation statistics, which were viewed as more reliable by market analysts.) Given the high levels of scrutiny of macroeconomic data, it is unclear to what extent data can be falsified.