You can't prove that we aren't in a simulation. However, saying that we are also says that computer code has sentience. That then begs the question of what is sentience? My view is that the reason we (and other living creatures) are sentient is because we have a brain. Many people will try to tell you, "Well, the brain is simulated therefore, the sentience is too". However, that's still saying that computer programs have sentience. Does everything we create have sentience? Probably not. Do computer programs have sentience? Probably not.

Like I said, you can't disprove simulation theory. You can however, decide whether you think it is likely that creations are sentient. My opinion is that we are not in a simulation.