I am writing to inform you that I read an interesting book. It is Richardson L. D. and Wolfe M. (2001) Principles and Practice of Informal Education. Published by RoutledgeFalmer.
I think that this book will give to answer your questions.
I'm currently working on teachers' informal learning.
The fact is that in the existing literature on informal learning, a multitude of informal learning definitions have been used interchangeably, (thereby somewhat overlapping each other). It doesn't really help to find a good way to assess the "still quite vague" concept (e.g. Meijiuni, Cranton & Táíwò, 2015).
For example: some scholars prefer to define teachers’ informal learning based on the stimulus (e.g. Eraut, 2000, 2004), while others focus more on the type of activity (e.g. Noe, Tews & Marand, 2013).
Callanan, Cervantes and Loomis (2011) corroborate that the definition of teachers’ informal learning is dependent on the authors’ interpretation of what it exactly comprises. Thus, as literature is not in unison on the definition of teachers’ informal learning, a myriad of partially overlapping definitions is used (Clarke, 2005). And of course, different ways of assessing the concept are used.
In a litterature review of teachers' informal learning definitions and measurements (soon to be submitted to a journal), we inventoried 32 unique instruments I can share with you if you still need it.
Most of them are assessing the differents activities teachers make to informally learn, not the "real learning",
Unfortunately, there is no single answer to your question, only a few leads.