Are we living in a simulation?

We might not. We might. Consider the possibility that it is possible for a simulation to run which would give a practically perfect experience to the simulated person. This is not so impossible to imagine given that not every person and experience would have to be simulated, just the ones you're interested in simulating. Further, the speed of the simulation doesn't have to be a one to one ratio to the real world, if it takes a couple years to simulate a second of the real world, that's not something the simulated person would have any reference to notice.

So if the simulated world is indistinguishable to one in the simulation, how then would it be possible for the person in the simulation to know they're in a simulation? There are possible ways. One is that there might be something to give evidence, such as a rendering problem, or pixelation. Another is that the simulated world might be given signs. Perhaps the person or people running the simulation might give some avenue of communication or confirmation.

It might be as simple as a testable code (see the recent Doctor Who episode about the subject) or it might be that the runner of the simulation considers themselves to be the god of the world. Try praying, and if that works, if you're an atheist, that's proof. If you're agnostic, it doesn't prove you're in a simulation, it just proves that you're in a simulation or there is actually a god, reducing the possibility to a binary proposition.

If prayer doesn't doesn't work, then that doesn't prove anything beyond the fact that you don't see a result at that particular time. For a theist, it doesn't even rule out the possibility that you're in a simulation. Of course, prayer doesn't even necessarily need to be the method available to communicate between someone in a simulation and someone running it. Perhaps the runner of the simulation has a fondness for a certain number that they'll use as a starting point to engage the simulated. Maybe it's some iteration of Pi. Get the necessary number of calculated digits and viola, you get to talk to your universe's creator. Maybe it's just a matter of using the right tools, much like breaking out of a virtual machine.

A theist might ask what God's purpose is. An atheist might wonder if there is a person running the simulation they're in with a purpose to the simulation. A non-philosophically inclined average person might wonder if there is a reason for the universe they find themselves in.

The question of whether we're in a simulation isn't really a new question, it is just an ancient question rephrased. The only difference now is that we have the tools to imagine possibilities we didn't before.

This is just the "god" argument in a different guise, so my reply is the same: Not until proof to the contrary.