You can hook up your computer to a random number generator or you could run the simulation on a quantum computer. Both would result in a non-deterministic simulation. My guess is that the simulation we are part of runs on a 5 dimensional, quasi-analogue, quantum, non-Von Neumann machine and we will never the chance to disprove that hypothesis from within the simulation.
Well if it's "non-Von Neumann" then it's not a computer by traditional definitions. Can we just use Turing machine for argument. A Turing machine makes more sense to me if we are going to proclaim we live in a computer simulation. If we did live in a computer simulation it would have to be Turing computable. I'm not sure reality is Turing computable.
Your random number generator idea does not take into account the role of an observer changing what is being observed at the smallest scale of measurement. This is the part that is strangely spiritual.
Also, you are confusing abstraction with reality. Your ideas about reality are not the reality they represent. The problem with your way of thinking is you are completely ignoring the analog nature of reality and consciousness. There is no digital clock pulse. In reality, everything is connected to everything else. The arbitrary abstractions imposed by measurements do not change the fact that everything is connected to everything else and parts of reality are happening all at once independently. There is no wall clock. The fabric of reality is relative. A computer simulation does not have this recursive semantics to its execution. There is no get-fetch-execute cycle to how matter behaves. Our minds are more like analog radio receivers than they are like microprocessors.
Here are two videos on this subject. The first has to do with linguistic philosophy of consciousness simulated on a computer.
This second video has to do with "idealism" versus "realism" and which one of the two is best supported by the scientific evidence. This matters because for reality to be a computer simulation there has to be a clock-pulse driving the get-fetch-execute cycle which there is not:
I once had an argument with someone who had dogmatic beliefs about Artificial Intelligence and coming Singularity which is always just around the corner in becoming real. Many years ago when I was studying artificial intelligence I was trying to prove human beings were no different than computers. I was trying to come up with a thought that was similar to the following computer program:
10 NOP
20 GOTO 10
This is an "infinite loop". And anyone who has study the limitations of computers, that is Computability Theory, knows about the Halting Problem.
Halting problem - Wikipedia
Since I was not able to create an infinite loop with my thoughts I came to the conclusion the human mind and our consciousness is not bounded by the Halting Problem, and therefore, artificial intelligence is impossible because our minds are NOT computer-like but something else.
The closest thought I could come up with to lock my mind into an infinite loop was this question: "Have you ever thought about what your brain is doing between thoughts?" Most people just laugh their way out of the infinite loop. Computers do not have a sense of humor. This is why I doubt very much the Universe is a computer simulation. There's just too much humor in the World for this to be a computer simulation.
Since I could not find a thought that would put my mind into and infinite loop I lost interest in AI. I no longer thought it was possible. And ever since I've been devoting my life to the study of how consciousness works and the true nature of the most profound spirituality we can experience here and now.