How to test if we’re living in a computer simulation

[C C]

While such has admittedly already been demonstrated to a very light or insufficient degree... 

What would make the hypothesis viable to begin with is if a fully "convincing to its inhabitants" simulation is ever created in this universe by us, posthumans, or ETs. 

So just don't allow that to ever happen, and our surrounding environment can continue making its little turn on the catwalk, posturing and preening itself as unique.

[Kornee]

One thing most sane physicists agree on is that a really real reality HAS to be based on existent quantum physics principles.
There could be no stability allowing anything like sentient beings, hypothetically progressing to becoming master simulationists (my pseudo-copyrighted innovative word I'm immensely proud of thinking up), otherwise.
Hence the notion what we perceive as 'spooky' quantum connections is necessary at the base reality level. Hence no giveaway clue at all of some cunning simulation shenanigans.