Testing? Comparison photos might give a good idea of what might be accomplished, but the lens is only part of that equation. Past experience with Samyang product suggests that the following should be addressed, at minimum.
Test for infinity focus. Tradition with Samyangs wider offerings has been that many lenses leave the factory unable to do fine focus much past 50m or so. Magnified live view should allow you to evaluate out to several hundred meters. I suspect that you will not find a copy of the 24mm that will have been set much further.*
Test for decentering. For this you can use the guide on this site or whatever method our prefer. My first copy of the Rokinon 14/2.8 was decentered.
How to Check Your Lens for Decentering - Articles and Tips | PentaxForums.com
Once the above have been dealt with, the best test for astro might be a few nights in the field.
Steve
* I had to nudge my Rokinon 14/2.8 a fair amount using sketchy DIY instructions found online. It may be easier to have Samyang do the adjustment.