An example of a test that's phenomenally complicated is, roughly speaking, to zoom in on physical reality until the resolution starts to drop https://en.wikipedia.org/wiki/Simulation_hypothesis#Testing_...

An example that's like magic is to communicate with someone outside of the simulation. Either someone in an adjacent simulation, or someone outside of the simulating system itself.

Basically, a simulation has to have rules and constraints. If we can get a fingernail into a crack we can expand it. Just keep pushing the limits until they break. We could even start the simulation version of SETI. That's a project that's testing the hypothesis that there are ETs out there we can communicate with. We could start a project like that based on the hypothesis there are Simulators outside of our simulation we can communicate with.