If we were in a computer simulation right now, how would we even know? Well, University of Bonn nuclear physicist Silas Beane and some of his colleagues have come up with a test that exploits a feature of said simulations; their need to be discretized. In the physical world, distances can be infinitesimally small, but not so in the world of computers where things need to have a limited set of values. Researchers use a lattice (i.e., 3D graph paper) to model how the strong nuclear force works over time on a femto scale. As the scale gets smaller and smaller, the simulation eventually hits a limit on how much energy particles can have. This phenomenon has an analog in the physical world called the Greisen-Zatsepin-Zuzmin (GZK) cut off, and Beane and company believe we can test to see if it behaves the way we expect from theory, or the way we’d expect in a discretized model like a computer simulation. If the latter is true, it would provide evidence that we are all stuck in a computer:

"The numerical simulation scenario could reveal itself in the distributions of the highest energy cosmic rays exhibiting a degree of rotational symmetry breaking that reflects the structure of the underlying lattice."

Possibly, anyway. The whole idea hinges on the assumption that whoever built our hypothetical simulation built it the same way we build ours. On the other hand, if our robot overlords have constructed a different way of doing things we might never notice that there is something wrong with the world.