The simulation is so perfect that people only realize it is fake by attempting to generate random numbers; since computer code is pseudo-random, everyone in the simulation produces the same sequence.
The Doctor (Peter Capaldi) discovers that the "real world" he is in is actually an incredibly detailed computer simulation created by an alien race known as the Monks. [S10E6] Extremis
Upon realizing he is a "simulacrum," the virtual Doctor uses his sonic sunglasses to record the entire experience and emails it to the real Doctor in the physical world as a warning of the impending invasion. The Vault & Missy The simulation is so perfect that people only
A series of flashbacks explains how the Doctor ended up guarding the mysterious vault under St. Luke's University: Doctor Who S10E6: Extremis - FilmInk The Vault & Missy A series of flashbacks