Don’t Quit, It’s Legit
If an authentic thing is said to pass an “acid test,” it is because real gold also had to pass one in the gold rush days. Gold’s non-reactivity to nitric acid distinguished it from lookalike iron pyrite, aka “fool’s gold.”
“The proof is in the pudding” derives from “the proof of the pudding,” which shortened “the proof of the pudding is in the eating.” This expression just meant you have to try food to learn whether its good. When the term began, however, “pudding” was a dish made of animal intestines stuffed with meat and other foods.
“Bona fide” also conveys that something is authentic, and is Latin for “in good faith.”