For a Warcraft character there's a lot of clues that they're in an arbitrary rule set. Horses vanishing when they go through a door . . . being able to hand someone an object but not set it on a table . . . not being able to drink milk without some lessons in the school of hard knocks . . . they're all clues. Then there's the random problems that come from errors in the code. Sometimes you can move across a flat surface . . . sometimes there's a little wrinkle and you're stuck. People or animals you've dealt with before act in bizarre ways or freeze. You can be moving down a ramp and suddenly fall through the solid ground. The bugs are a bigger giveaway than the deliberate design omissions.
So what bugs make it look like we're in a simulation? Well, there's light. Sometimes it's a particle. Sometimes it's a wave. Nobel prize winners wave their hands and gibber trying to resolve this. But that's exactly the kind of glitch you get when developers steal legacy code from two different applications. Verse A had light-waves, Verse B had photons, and our world behaves according to which one a particular piece of code came from.
Then there's the speed-of-light limit. Integral to physics? Or just a simple barrier the devs threw in to keep us from peeking at under construction areas? ("Nerf light!" celticdragonfly says to this.)
Biology is full of bugs, no pun intended for once. The boot process for life is hard to explain. Evolution keeps producing errors despite millions of iterations. And human psychology . . . well, is that a bug or a deliberate design feature?
I mean, if you have two possible explanations for how the human mind works:
1. A product of evolution intended to maximize healthy offspring
2. A scenario generator optimized to produce entertaining conflicts for spectators and role-players
. . . which one fits the observed data better?