You cannot through experiments prove that the law of conservation of energy holds in all cases. Indeed you cannot through observation prove that any law of nature holds in all cases. But you only need a single counterexample to show that a law doesn't hold in all cases. This is exactly the rationale behind the principle of falsification. You cannot prove through observation that all swans are white. But you can prove that not all swans are white by a single observation of a black swan.

The bottom line is that it is possible to devise experiments or observations which would give different outcome if the theory holds and if the theory doesn't hold.

But you cannot devise an experiment which would give different outcomes if the universe is a simulation versus if it isn't. More specifically, you cannot imagine an observation which would prove that the universe isn't a simulation. Therefore it is not a legitimate scientific theory. At best it is a fun thought experiment.