A simulation is per definition:

a: something that runs inside some hardware

b: nondeterministic

c: limited in level of detail

d: needs a frame rate

e: has a (limited) resolution

Considered the fact that a simulation runs inside a restricted environment (hardware specs, RAM, storage, resolution, etc) and can only works with a calculate the outcome given input parameters, a simulated intelligent being (SIB) would be able to find the cracks in the simulation. How? Very simple.

Let us start with the notion that it's actions are not dictated by the program, a simulation means you let the program run and see what evolves over time. So anything a SIB being would do is not dictated nor controlled by the program or the supervisor of the simulation.

So if the SIB starts exploring it's world and start digging, climbing, diving and flying it would soon find out that there is something weird going on since there are no details beyond a certain level. In a real world, if I put my spade only a few inches inside the ground, I can find a whole microkosmos over there, filled with tiny creatures, roots, seeds, sand particles, pebbles and perhaps some items someone once lost or tossed away. In the pebble I can discover fossils that are consistent with other fossils found in similar rocks. And if I decide to take a sample and put it under a microscope, I might find an abundance of organisms like bacteria there, living and multiplying. Wherever I decide to put my spade in the earth, I will find similar results but always unique. Not ever a single exact repeating pattern to be found.

In a simulation, the level of detail is restricted by the hardware and patterns are repetitive and predictable. If all particles of an entire universe would have to be simulated including their history, the hardware needed to simulate the universe would have to be larger than the universe itself.

And today, humans explore their world in great detail, scrutinizing everything. If we point a telescope at some empty spot of sky, with modern telescopes we find there are millions of galaxies visible there, all red shifted and speeding away from us. We can do experiments on the sub-atomic level and find the particles that make up the stuff we're made of.

Everything we find, from Quantum mechanics to cosmology, it is all consistent and really nothing at all gives away even the tiniest clue there is something fishy here, which means either it is real or the simulation keeps track and stores the information of every single particle in the universe… from the beginning (whatever that was) until now. That is one hell of a simulation.

Now consider what a simulation actually does. First all objects must be defined at the beginning, they must have properties. For a particle, it means intrinsic properties like energy, mass, spin, position and momentum. But here is the catch, the position must be defined within the available computational space of the simulation. If I make a simulation for a computer screen, the position of each pixel is within the range of 1920 x 1080 pixels. I f I have to store this information, I only need 22 bytes (2*11 bytes). And this position is update every frame. In VR simulations, this frame rate is at best 90 frames per second.

If it's a 3D simulation within a virtual space, I have to pick the size and the resolution (accuracy) myself but things are getting quit bulky here. Now imagine what hardware and memory it takes to calculate the trajectory of only a single photon travelling through the galaxy when the simulation is done with a clock frequency equal to the Planck time (that 5.391247×10*43 s faster than 90 frames/s) , the position of the particle needs to be stored with the resolution of the Planck length (1.616255×10−35 m), within a simulated 3D space that is expanding(!) at an enormous rate (estimated at 73.5 ±1.4 km/sec/Mpc), meanwhile calculating all interactions with all particles it meets, and at any moment, the gravitational pull of all other particles.. in the entire universe! Take my word for it, that is impossible, even for one single particle.

And yet, that is the sort of simulation we're talking about since this is what we can find out about our world and is consistent in every detail.

In other words, if you know just a little bit about programming and creating simulations, you know that the idea of living in a simulation is just the product of peoples minds with a lot of imagination and a total lack of knowledge.