The place of publication is legally required to appear on the back side of the title page. That's how you tell whether it's in AE, BE, or (as possibly in this case) Canadian English, which follows BE in some respects and AE in others. Amazon should actually include that information too, and the different editions will have different ISBNs, I believe.
It has long been customary to change the spelling when a book crosses the Atlantic, even though there's not really any logical need to do so. So readers are accustomed to words like 'gray/grey' appearing in the spelling familiar to them, no matter what the setting, just as German readers would not expect a word like 'Fuss' to appear in a German book just because the speaker was Swiss. After all, you wouldn't want constant switching back and forth if you had characters from different countries, would you?
'No, it isn't, it's gray!'
But actual vocabulary changes are really a dumbing down of the language, as we discussed when the Harry Potter series came out in AE and made the kids sound American, not British. Such heavy-handed editing deprives readers of the chance to learn about cultural differences -- it's as if C. S. Lewis had been marketed to American kids as 'The Lion, the Witch, and the Closet.'
Is 'armoire' really not used in BE? I would have thought that almost anything French would be old enough that it must have entered the language via Britain. But if it sounds out of place, then it's a research error on the part of the author. That's the risk writers take when they set stories in other countries.