You can kinda guess the world is real because of the CAP theorem. Hear me out.

1. The CAP theorem says a computing system cannot perfectly have all 3 of: Consistency, Availability, and Partition tolerance (division of some parts of a distributed system from another). We'll assume this is true and somewhat dubiously assume this applies to any simulated universe
2. Availability is a necessity. A simulated universe that suddenly starts lagging or buffering would mean the jig would be up pretty quickly. You'd probably want a distributed system that can spin up new computing instances instantly, but that brings up issues with partitioning....
3. But lack of partition tolerance would make it pretty obvious that the universe is fake, because some parts of it would be inaccessible. So can't sacrifice that.
4. Therefore, the only thing left is consistency. A simulated universe would need some kind of inconsistency. In a web site, this might mean content is available to users in some areas but not others. In a simulated universe, we'd expect people in some areas to have a different experience of objective reality than others. But there's no evidence of this ever happening, unless you wanna go down some Mandela effect rabbit hole.
5. That leaves us with the conclusion that the universe is not a computing system at all, but rather a thing in itself. It doesn't need to stay consistent because it is consistent fundamentally.
6. Also, let's just ignore relativistic speed limits and quantum mechanics entirely.