This is an interesting hypothetical that came up in a game I’m playing. We ended up not rolling because it wasn’t important enough, but if deducing someone’s hidden feelings was necessary, and important enough to be its own throw of the dice instead of being rolled into another social test, what would you roll?
Will, because it’s a matter of social intelligence? This is my instinct.
Perception, because you’re spotting a reaction?
I could see observation argued because you’re “spotting the hidden” but folks are pretty strict about that only testing vs the four listed skills.
Various skills could work depending on your means, like using etiquette to engage them in polite conversation while obliquely sussing out their opinions.
You might say that they should should roll falsehood against an Ob of your will, but this isn’t telling a lie exactly… so persuasion perhaps? But to make the cagey character the actor feels like it’s assuming everyone plays with their heart on their sleeve unless they make significant conscious effort to do otherwise.
My understanding is that outside of magic (aura reading) and special traits (like manhunter), determining another person’s true feelings through observation is deliberately left out of the dice rolling systems.
Common techniques include the GM just telling the player, “This person is lying to you,” or, “This person feels like X,” and then asking the player what they do about it; letting the players act in the dark; and players getting the other character to open up to them through Soothing Platitudes, Falsehood, Ugly Truth, Interrogation, etc – or just through exploratory conversation (Saying Yes, like).
My general read is that Burning Wheel tends to assume those kinds of things are either volunteered, or forced out. So a character might use Soothing Platitudes to give the other person to keep talking, or Ugly Truth to draw them out, or Conspicuous to force them to be surprised enough to leave their feelings on their face. But generally, a character reveals when they want to unless you can find a way to make them reveal.
I agree that there’s spy genres and other genres where that’s not the case, but generally BW shies away from them.
But yeah - Manhunter is the trait if you want to do this passively and non-magically.