Mod-Laws of Physics
Try to change the laws of physics, and verify if we are at base reality or not.
I have one very observable thing - the laws of physics. If we were at base reality, arguably, the laws of physics would be part of it, as an object, which we can change. For now, we have zero evidence that someone would have changed the laws of physics. So, the idea is to run experiments to try to change the laws of physics at large. For example, change the Speed of Light, or something else that we can measure very precisely.
Of course, we can predict the consequences of changing the constants. For example, changing gravitational constant would result in change of planet orbits and densities of matter (e.g., intensification in the number of supernovas). This could be an avenue for observational evidence.
On the other hand, if it is the fact (which it seems to be) that we don't see the laws of physics as separate modifiable objects, then it arguably suggests that we are not at base reality.
Your critique is very welcome -- would it work, or why wouldn't work.