You might consider a survey of participating teachers as an additional source of information. In addition, you may wish to develop a workshop quality checklist, based on the principles of adult learning theory. You may need to do a little background research to come up with a workable draft. Vet it to some respectable PD folks that you know. After getting their feedback, pilot test it by using it to grade the various PD activities. You could also use a Likert-type scale for some of the items (e.g., "To what extent does this workshop ...?") The principles of good adult instruction are well documented. Try the Association for Supervision and Curriculum Development (ASCD) as a source (http://www.ascd.org/).
To really answer the validity issue, you will need an independent external measure of the same variables to corroborate your findings. The HRI Classroom Observation Protocol might be a good source for that. The validity question is: To what extent do the two independent measures of PD quality yield similar results? Correlational evidence in the .75 range or higher should be good enough to establish validity if you have evidence that your measure is addressing the correct (most important) dimensions of adult instruction.