Emocards provide a non-verbal method for users to self-report their emotions. Flash cards or single sheet of a paper.
At the end of every task / time duration, ask the person to pick one of several cartoon faces that identifies how they are feeling about their interaction.
Quick and easy
No training needed
Can be added to any user study
Only granular to the level of frequency that the person is asked to choose (typically the task).
Interrupts the task if asked during it
Desmet, P.M.A., Overbeeke, C.J. & Tax, S. J. E. T. (2001). Designing Products with Added Emotional Value: Development and Application of an Approach for Research through Design. The Design Journal, 4(1), 32-47.
References describing the method