So, there are two Pis. Let's call our Pi x and a simulated Pi y. Let's also assume that y is equal to the first 3 decimal places of x. IE y=3.141
y is an approximation of x. This means that the decimal expansion of y stops at some point. Since the decimal expansion stops at some point, y is a fraction. In this case, y=3141/1000.
Now, that means* there can't be a proof that y is a transcendental number. IE, it cannot be shown that y is a solution to some polynomial equation with integer coefficients. This means there couldn't be any expression like:
(z-a1)(z-a2)(z-a3)...(z-aN) = 0
which has Pi as one of the solutions. A familiar polynomial might be z^2 - 1, others are z^3+2z+z, z^n + 3 where n is an arbitrary member of {0,1,2,3,...}. That kind of thing. x can't be a solution to one of those.
But y can. In fact, y is, 3141z - 1000 has y as a root (set it equal to 0 and solve it). This means y is not transcendental, since it is a root of a polynomial with integer coefficients. More generally, if y is a fraction a/b, then bz-a has y as a root. Thus, every approximation of y will not be transcendental.
If we are in a simulation, then y isn't equal to x, since the simulation can't be of infinite size (no infinite memory, infinite memory required to specify Pi exactly). But the reality has to be quite similar to the one with y=x, so y would be an approximation of x. And we're in the situation I described above.
This gives us a means of distinguishing simulated and non-simulated universes while we're in them. A sufficient condition for not being in a simulated universe is:
there is a proof that Pi is transcendental. In our universe, there is such a proof.
I appreciate the attempt to frame the proof as being equivalent to some bundle of sense data, but it really isn't a bundle of sense data or a representational artifact. It's a fact, in our universe Pi is transcendental. The 'sense-data' of the proof isn't equivalent to
the proof as a demonstration of a fact.
So proof isn't just a bundle of sense data, it's a demonstration that something is true about our universe. If 'pi being transcendental' isn't sufficient for this, surely 'no one can square a circle using a straightedge and compass'
is, since it's well within the bounds of a simulation to allow people to play about with axiomatic systems in mathematics and for them to draw with compass and straightedge.
If you want to frame it in terms of
visual sense data, it's still possible: in a simulated universe, no one could
see a valid proof of
Pi is transcendental. And conversely someone
could see a construction in the simulated universe that allows squaring the circle. This still allows discrimination between simulated and non-simulated universes based off of the incapacity for anyone in a simulated universe to
see a valid proof that Pi is transcendental, since such a proof couldn't exist*.
*unless the axioms of geometry were inconsistent, which they aren't.