If the Universe were a big simulation running on a super fast computer built by an advanced civilization, it would be possible, in theory, to determine if this were the case by coming up with a system to detect an outside of the Universe time reference.
A simulated system, running on a virtual computer, will have an internal clock whereby it marks time.
If that simulated system has the ability to detect a time reference that is external to the simulation, the simulation can not only conclusively prove that it is a simulation but also at what speed the simulation is occuring.
When I say “time reference” it does not need to be a clock as you and I would understand it.
The time reference could be the orbits of electrons, or the radioactive decay of a particle.