That's not true at all. If someone gives you a gold bar right now, would you be able to tell if it's real or not? What if, for example, it's steel on the inside

That's not true at all. If someone gives you a gold bar right now, would you be able to tell if it's real or not? What if, for example, it's steel on the inside coated with a thin layer of gold. Or the gold has a low purity, etc. This is why there are specialists (who are very expensive) who can use special tests to check if it's real (and even then it's not 100%). It's easy to tell if the Bitcoin that you just received is real or not. (Definitely much easier and cheaper than with gold).