system
Given the following post, respond as if you're commenting on Reddit:
human
[Mistborn] How does someone figure out which metals they can use, if any?

I've only read the Wax and Wayne books, so maybe this is explained in the earlier Mistborn books, but how does someone discover if they are an Allomancer or Ferrochemist and which metals they can use? If I'm a Pulser I can eat a pinch of cadmium and slow down time, but if I'm not I'll just get cadmium poisoning and lose my kidneys. So how do people safely test which metals they can use, or if they can use any at all?
gpt
In the Final Empire, the world that existed before the  Catacendre , only nobles and half bloods had allomatantic abilities.  Nobles where beaten as children until they “snapped”. That is, experience enough trauma to crack their souls and let in the investiture, the name of magic in the cosmere universe.  At that point they would be given a flask of all the allomatic metals and asked to burn them.  At the time only ten metals were known: iron, steel, pewter, tin, bronze, copper, zinc, brass, atium and gold.  If you could burn any you were a misting, If you could burn two you were a mistborn.  The super low levels in the vial wouldn’t be a risk and most of these metals arent toxic to humans in even in relatively high does.

Harmony changed the way snapping worked, ended the mistborn, may have reduced the power of allomancers  in general, and gave a better explanation of the metals expanding out to the full sixteen seen in the post-caracendre basin that Wax and Wayne live in.