This exercise of assessment of trainers, is it for the external trainers or internal trainers? Secondly, why do you wish to do that? What deficiency did you find when you were selecting the trainers through earlier method? Above all, generally it is trainers who assess the knowledge of the participants or employees. But then you wanted to assess the trainers. How many managers are capable to "assess" the knowledge of a trainer?
Training is conducted to bring the positive change. Instead of measuring this change, why you wish to get into the assessment of the trainers?
Your post needs clarification. Therefore, please clarify further.
From India, Bangalore