A common problem implementing PBL training is that the teachers don't know what to look for while observing others. When observations are unstructured, then reflections and discussions can run flat or be all over the place, thus minimizing the outcome of the training or co-teaching.
We use this observation sheet (located at the end of the article) to structure our observations of PBL lessons. The observation period can last either long-term (over the length of the whole PBL project), mid-term (for a phase within the project), or short-term (one individual class).
I have included the article with this observation checklist to give you all a clear background on its uses and merits.