Buddha said that he never taught anything of his own. Socrates also said similarly. In other words, they told people 'what exists' just as it is. Their teachings didn't come from their thoughts or ideas but what exists in the world.

To know what they taught, look at their life activities. Even though people didn't treat him very well while he was alive, Buddha had endless love and mercy for people, and tried repeatedly to awaken them so that they can save themselves.

Because Buddha tried to help awaken people non-stop for 45 years and because his teaching is still accurate by today's view, I know that he would never lie and tell people to practice something wrong. That's why whatever is true or not true, among what Buddhist monks claim is Buddha's teaching, can be proven by the facts. If it's Buddha's teaching, it will have all the evidence and no falsehood.

I've also met many Buddhist monks, so if you're still not convinced, I can show you as much evidence as you need that they don't realize anything about the true teachings of Buddha. As long as you give me questions I can give answers. One simple fact is that if monks know Buddha's teaching then they should have some evidence that they learned directly from the master. But that is impossible since Gautama Buddha died 3000 years ago. So I'm not claiming I know Gautama's teachings but my reasoning and experiences as explained here are only that which I've seen with my own eyes and verified with substantial proof.