The only thing that distinguishes the concept of "reality" from that of "illusion" or "simulation" is consistency. If a simulation or illusion can maintain perfect consistency for the entire interval in which it is experienced, without the possibility of experiencing anything else to compare it with, then it is reality.

We don't know if we are living in a simulation or the true reality because we can't experience anything before birth or after death. And even memories of past lives would be taking place within our current experience, so they can't be used as evidence for anything outside the current experience.

I came to the conclusion that what gives reality this persistent sense of consistency for all physical observers, regardless of their epoch, density, or location, are the invariant ratios and quantities (the universal constants) that their subjective measures of each aspect of reality (space, time, motion, density...) must satisfy, leading to the notion of a fundamental, unbreakable uncertainty or quantization underlying all reality and enforcing self-consistency across space and time.

What we usually consider to be the laws of nature (thermodynamics, gravitational attraction, kinematic motion...) follow naturally from the enforcement of the invariant conditions and physical constraints represented by the universal constants, which all physical phenomena, regardless of their nature, must satisfy in order to exist.

Thus, the universal constants (c, G, h) are the ratios and quantities that ensure that the universe has perfect causal consistency through space and time, even though each observer experiences different distances and durations and perceives the universe as having more or less of this or that due to its motion (SR) and its density (GR).

If we are living in a simulation, the "checksum" represented by the universal constants is what prevents us (or anything else) from telling otherwise.