How would you know? Based on what we know of simulations in this reality, which has rules designed by whoever created it? If you're programming a simulated universe and don't want its inhabitants to know, it seems like a pretty obvious mechanism to try to enforce that would be to make the rules so that it seemed like "leaking" information into a simulation was impossible, even if the rules of their "outer" reality made it impossible for them to fully prevent it, only dissuade us from looking into it.
This is kind of why I mostly find the discussion of whether we're in simulation to pretty quickly reach a point where it stops being interesting even from a philosophical perspective. I don't really see how you can differentiate between fundamental properties that we observe that reflect the "real" universe where we're just a simulation and fundamental properties that explicitly designed for the purpose of the simulation itself and may not actually reflect the "real" universe. We might as well ask if the OS our simulation is running on has a toggle for dark mode or not.
There's no test that can make us convince ourselves that it's a simulation or not. The real question is if it even matters. We are just as real if we're not a simulation, or are a simulation.
Belief is simulated knowledge and we exist within our believed world...so we are in a simulation of some kind, but we don't know if we are also in "the" kind.
They can just simply pause, delete, reverse, or make us outright ignore anything. They are literal god and hold ultimate power over us, unless they are willing to deliberately let things happen.
