I don't think we can physically test such things, from my understanding, the core concept of this idea is that if we are in a simulation, then if we are able to simulate an accurate universe, that means we are a simulation ourselves. I can't be the only one seeing the problem with this?
This seems backward to me. Wouldn't simulating a whole universe inside a simulation of a universe be the kind of thing that would break the simulator?
we assume they are simulating a universe in equal size to their's, for all we know, the perceived universe we see could simply be a giant cube-map texture, being fed from their universe, or their universe is perhaps millions upon millions of times bigger than ours. so we could be a relatively small universe by comparison to there's. remember that we haven't even left our solar system(the computer they are using might be billions of cluster's of servers, think of each solar system, planet, or maybe even atom, could have it's own dedicated processor).
we truthfully have no possible idea how they could be doing this, hell, their universe could even operate completely differently than ours, and we are making assumptions that they are trying to simulate their own universe, for all we know, we could be some huge MMO to them.