>>34068391>Celestia tested for
She didn't know Luna well enough for the test to work. All her "test" did was satisfy her that Luna wasn't a ponycidal maniac.
The journals in conjunction with her not abandoning the moon colony proved that she hadn't been Discordified.>because Twilight probably would've noticed the magic on her
Twilight only ever sees Luna in the dream realm, you can't just assume that spells on your physical body (of which Luna also has none) would transfer there. That could only be proven false on NMM's physical return.
On the threat of a tantabus, this was never a well thought out possibility. Forthwith admitted he added it later in the story, following pic related.
There's not really any way to test for it as described, until NMM is released or potentially ever. Basically only Luna could test for it, who is under its influence, or a plot-breaking device like the EoH.
The other interpretation of the golem problem was that Luna was a construct for Twilight to interact with and to be discarded on NMM's return resulting in the "Twilight's Fate" chapter. Its purpose would obviously be to hide NMM's ponycidal temperament, but is proven false in the scene where Luna loses her shit after discovering Celestia's plan to "kill" her.