A third possibility: Humans aren't in general capable of accurately reflecting on their preferences.

Humans are obviously capable of perceiving their own preferences at some level, otherwise they'd be unable to act on them. I assume what you propose here is that conscious introspection is unable to access those preferences?

In that case, utility functions could potentially be deduced by the individual placing themselves into situations that require real action based on relevant preferences, recording their choices, and attempting to deduce a consistent basis that explains those choices. I'm pretty sure that someone with a bit of math background who spent a few days taking or refusing various bets could deduce the nonlinearity and approximate shape of their utility function for money without any introspection, for instance.