Visual signs? The color. I don't think that color is possible naturally at all. The treatment didn't imitate something, just created a new pearl color along with a fashion fad for brown (one of those 'new black' waves) a couple of years ago.
Natural 'brown' would be a warm black - very dark with sight coppery overtone. Not at all similar with the treated. before the 'chocolate' treatment craze, I've never, ever heard of those brownie pearls being prised for their color (could have missed that, but wouldn't bet on that). Much as I now, the brownish black pearls tend to be dull too.
That's as much as I understand of the story, at least.
There sure are folks posting here who know this first hand. Not me...
Btw.
That is one magnificent ring in the picture! Yours? Your work? Any chance to see it from the above?
Awesome platinum work; reminds me of a certain workshop...