You know I can remember watching Westerns on TV and a bad guy would taste or slightly bite a gold coin to confirm its authenticity. If I can think of the name of the movie I'll post.
Another risk... After one's had a taste, it can be really difficult not to eat a little bit of the coin. I've usually been able to limit myself to only eating small fragments (25% or less)... For this one, I had a little bit more: Although I have not read the comedies (i.e., The Birds and The Wasps), it's my understanding that the playwright Aristophanes discusses how the Athenians, being ever wise and restrained, made their coins last longer by sucking on them like lozenges instead of taking big bites.
I believe the theory is that good metal coins when bit will leave a mark in the gold but plated fakes will not. I believe many (older?) people could pass a blindfold test and identify by mouth Sterling silver, plated, copper, or stainless steel spoons.
I'd imagine that depends on the thickness of plating... With modern methods of (especially electroplating) the plating may be too thin to sustain a dent-by-biting (i.e., it would have the effective hardness of the metal underneath). But on ancient fourrées, the thickly plated / foil-wrap ones, I wonder if the bite test would've worked. I haven't looked closely at gold ones, but the silver often seems thick enough I'd imagine it might be dent-able by tooth. I'd be fine sacrificing a plated denarius to test it out. I just wouldn't want to sacrifice my teeth if the metal wins.
I was of the understanding that fake gold coins would often be coated lead to give the coin that "heavy" feeling, and you can easily bite into lead...I've proven that with many lead sinkers while fishing.
On a related note, if you hold a copper penny and a silver dime in a V shape between finger and thumb and stick your tongue between them, you will get a sour taste as they form a battery (single cell)