With very little experimenting an AGI instantly can find out, given it has unfalsified knowledge about laws of physics. For nowadays virtual worlds: take a second mirror into a bathroom. If you see yourself many times in the mirrored mirror you are in the real world. Simulated raytracing cancels rays after a finite number of reflections. Other physical phenomena will show similar discrepencies with their simulated counterparts.

An AGI can easily distinguish where it is: it will use its electronic hardware for some experimenting. Similarly could it be possible to detect a nested simulation.