Computer simulations for games often use optimisations.
If the player is watching stuff, it is modelled realistically. But if no one is looking, a cheap approximate solution is used instead. This trick saves a lot of CPU cycles.
To discover if we are in a game, we need to look for an optimisation fingerprint. We need evidence that stuff behaves differently when no one is watching.
In our universe, the double-slit experiment shows that light looks like a wave when no one is observing closely - but starts looking like particles when we take a closer look!
It's obvious that modelling every bit of light as a particle is way expensive. So the universe switches to a more optimal wave representation to save cycles.
Clearly then, we are in a Matrix.