Try this effective AND manageable peer evaluation sheet for fishbowl style socratic seminars. Each student is anonymously assigned another student in the socratic seminar to evaluate.
This checklist has worked the best out of all the different methods I have tried. It collects all the data the teacher will want to see on each student (how many times did he/she speak, ask a question, clarify, interrupt, distract, etc.) but since all the evaluator has to do during the seminar is make checks, it does district them from the conversation too much.