Joy is a term related to Psychology, then how can we evaluate the joy of all students at a time. Can we evaluate through result or feedback or any other method?
Anyway, take care of adding interesting but irrelevant information to the lecture. That seductive information could redirect attention in a non-wanted orientation. The 14th chapter (Teaching by Priming Students' Motivation to Learn) from Adolf Meyer's (Learning and Instruction) book makes a good analysis of such questions. It seems important, for example, to differentiate between emotional and cognitive interest, where the last is achieved through structurally coherent texts, in which the underlying structure is easily determined.
One factor you may want to measure is whether the class activities are interesting enough that students take them home and keep engaging with the material. Without assigned homework, that is.
Joy can be at the individual or group level. The joy of creating their own Bulletin, combining Field Study with theory, creating a short film, student seminar, Projects oriented to student's capabilities and interest but related to the subject of study, Games enlightening theory and so on can multiply joy in learning.
as i have been involved with teaching for almost the last 20 years, i can strongly say that how close the teacher is to his students is the main parameter to evaluate how the students enjoy but at the end, it's still the success with high marks is more important for students and definetly, their parents