The more I think about it, the more I think TenSoon may have just outright lied to Vin when he said Kandra weren't affected by emotional Allomancy.
If I am a Kandra, privy to the knowledge that Allomancers with sufficient strength in Brass or Zinc can control you like a puppet, I'd be inclined to tell them that, "Yea, that emotional Allomancy stuff? Doesn't work on Kandra so don't even bother trying."
So, my players have discovered a simple, fairly fool-proof way to test if someone is or isn't a Kandra. They just have someone Sooth or Riot a specific emotion, and see if the person of interest can tell which emotion they're manipulating. I... Don't really want it to be this easy to discover Kandra. Does anyone have a way to foil this?
Do they have any way to know this in character? This sounds like something that would definitely be a secret and if they haven't uncovered it in game my response would be along the lines of "Sorry, your character has no idea that Kandra aren't affected by emotional Allomancy."
That aside, Zinc and Brass are supposed to be subtle. The narrator has to spend 2 complications to have the target realize they were targeted at all to say nothing of them noticing which emotion is being toyed with. The vast majority of people, not just Kandra, aren't going to be able to answer that question correctly.