So you have to start with say, a universe with
no constraints at all. Impossible to imagine. What would then be the purpose of a simulation at all? There wouldn't be any, because no constraints means everything is knowable instantaneously. A universe with no constraints means that nothing really ever happens.
But supposing there were some reason to run one in a universe with no constraint (impossible, since even having a reason to run a simulation is itself a constraint), would the simulation have any constraints at all? I don't see a reason why. Infinite compute available to simulate everything all at once would be trivial.
So we can then deduce that a universe in which a simulation is running has constraints. We do not know what these constraints are, but we can be certain they exist.
Some of the constraints within the simulation, unbeknownst to the entities inside the simulation, are the constraints of the universe in which it is running, and not constraints imposed by the simulation itself. Which constraints are these? Well, it would stand to reason that these constraints would appear to be fundamental. Maybe in ours it's the speed of light demonstrating the compute limit of the simulation. Maybe our simulation exists to better understand their constraints, and so something much more difficult to understand is indicative of their constraints. Maybe those constraints are reason itself. Maybe the fact that the process of the universe unfolding is indicative of their constraints. Maybe something else, maybe I'm wrong and its something that doesn't appear fundamental at all, but it doesn't really matter. Information leaks into your universe from the universe in which it exists. No matter the nature of the "bare metal" universe in which your universe is running, there will always be some aspect of yours that is reflective of the higher level one, and there will always be a way to tell that you are in a simulation. All that is needed is to examine the details of how yours works, hand wavy and much harder than it sounds, but theoretically it is possible. You cannot create a simulation in which the entities inside cannot possibly know they're in one, and in which nothing at all can be known of the higher reality.