I'm going to type out all the dialogue from that room, though, in the order I think it's supposed to be absorbed.
First, there's the dialogue everyone sees (unless they literally run straight to the fucking door, as way too many players actually do):
Gladiolus: "Remember how nervous we were in front of King Regis the day we left?"
Ignis: "That was the last time we were in here."
Prompto: "Yeah. They even let me in that day."
Noctis: "Feels like forever ago."
Also, in
this video (around 6:03:05) you can see/hear this bit of discussion about the paintings and prophecy in general if you examine one of the wall murals:
Ignis: "Do the paintings of the prophecy still adorn the walls?"
Prompto: "Yeah. Intact...and amazing."
Gladiolus: "We'll witness firsthand whether their story comes true."
Noctis: "Oh, I'll make sure it does."
And this dialogue when examining the other wall mural (as seen at 32:18 in
this video):
Noctis: "Hmm. Guessing Ardyn was meant to appear in these, too."
Gladio: "Hmph. Never would have guessed that guy for royalty."
Noctis: "He's been living a long time. In a world he could share with no one."
Then, if one sticks around a few seconds (again, instead of running right into the damn doors):
Ignis: "I recall a young prince leaving his mark on a painting. Is it still there?"
Noctis: "I don't think anyone could spot it. Had one of my attendants cover it up pretty good."
When examining the painting that is presumably the one in question --
Prompto: "So Ardyn...is the darkness of the legends."
Ignis: "Ironically, since he was to be the King of Light."
Noctis: "And only I can deliver him from his own darkness."
Prompto: "My buddy's gonna write the last chapter of the legend. So cool."
Finally, again, if one bothers to walk around for a moment:
Noctis: "I always hated this room as a kid."
Prompto: "Yeah? And why's that?"
Noctis: "Coming here always made my dad all serious and sad."
Ignis: "It no doubt reminded him of the fate his son would come to bear."
----
So, there we have it. Ardyn confirmed to have been the Chosen King.
I'll try to find a Japanese recording of the painting to see how he was described there, but I'm confident we can count this as confirmation.