So, it comes down to this.
1 Take the starlight spectrum, measure with a grating the wavelength.
2 Take the starlight spectrum, measure the frequency of a spectral line,
to do that possibly mix with a know monochromatic source,
difference frequency for some spectral line in radio range hopefully.

We know: frequency x wavelength = propagation_speed

We multiply.
The propagation speed we find we divide by c (Einstein's)

If we see a difference from 1 we claim the students are all on summer holiday,
and we are doing a double blind test, and promote the professor in question
to a NASA payed job.

We declare Einstein is a genius.

LOL