https://wiki.tfes.org/Universal_AccelerationAs the article states, there is no definition of a "resting" observer within Special Relativity. However, we do have a definition of a moving observer. In fact, there is a moving observer who always has the same velocity:

Photons.

The velocity of light is a constant and no matter into which moving coordinate-system you transform the movement of the photon, it always moves at the speed of light.

Why is this interesting?

Because of the Doppler-Effect. The wavelength of an observed photon depends on the velocity of the observer.

https://en.wikipedia.org/wiki/Doppler_effectIf we are looking at starlight, our velocity as an observer relative to the starlight depends on at which angle we are looking up into the sky. Light coming straight at us from above has the largest blueshift, while light coming at us from the horizon has no blueshift.

The same goes for the light of a light-bulb. Let's assume that Flat Earth is moving upwards and that we are looking upwards to a light-bulb. That means, the light of the light-bulb has a blue-shift relative to us. Now, if we look at the same light-bulb from the side, the blue-shift disappears.

How big is this effect?

If Flat Earth experiences a constant acceleration of 9.81 m/s², after 1 year = 31,557,600 seconds it has a velocity of approximately 3*10^8 m/s. That's light-speed. Now, of course we are not moving at light-speed, but this simple estimate makes it reasonable to assume that Flat Earth has been moving at a velocity close to the speed of light since a few years after its creation.

For simplicity's sake, it is a reasonable estimate to assume that Flat Earth has a velocity of something in the ballpark of 10% of the speed of light.

The formula for the Doppler-effect is:

f = f0 * (1+ v/c)

if we are moving towards the emitter with a velocity v. As wavelength is the inverse of frequency, the corresponding formula for wavelengths is

lambda0 = lambda * (1+ v/c)

For the measurement you will need:

- a shining lightbulb

- transparent colored material

You hold the colored transparent material between the lightbulb and your eye. The human eye can detect wavelengths in the range of 400 nm (violet) to 700 nm (red). The colors are as follows:

https://en.wikipedia.org/wiki/Electromagnetic_spectrum400 nm - violet

450 nm - blue

500 nm - turquois

550 nm - green

600 nm - orange

650 nm - red

700 nm - red

If Flat Earth has a velocity of 10% of the speed of light, then the wavelengths you see can shift up to 10%. And that means that the human eye would see a noticable shift in color, depending on whether you look at the colored light from below or from the side.

If you are worried that the effect is too weak for the human eye, fear not. There are a myriad ways to measure this, from simple prisms to the spectrometers built into the digital cameras used by professional photographers to apps you can download for your smartphone. (just google "app smartphone measure spectrum")

So, what are you waiting for?

Universal Acceleration is just one lightbulb away.