How can experimental compliance (willingness of participants to participate) be measured?
You might look for examples in the Witt, Donnellan &amp; Orlando (2011). They tested subject motivation in a psychology subject pool to see how it varied among groups. There may be follow up studies. Looking through similar studies will give you an idea of the kinds of things people use to assess. This is not a standardized line of research (yet).

Witt, E. A., Donnellan, M. B., &amp; Orlando, M. J. (2011). Timing and selection effects within a psychology subject pool: Personality and sex matter. Personality and Individual Differences, 50, 355-359. doi: 10.1016/j.paid.2010.10.019