Retrospectives can be complicated meetings. Done correctly, they can provide immense value. Done poorly, they can be a show that provides negative value. A well run retrospective requires more than just going through the motions. Several things contribute to a successful retrospective; however, in my experience the key ingredient is trust.
I expect Fred George (all the way on the right, 2nd from the top) would probably agree with me. Fred is the only person I've ever met that would start each retrospective by asking everyone to write down, on a scale of 1 to 5, where their trust level is at.
A retrospective without trust is pretty worthless, so measuring trust at the beginning of the meeting definitely makes sense. If the trust level is ever below the acceptable level (assuming 5 is complete trust, anything below a 4 is potentially a problem) then the meeting doesn't proceed until a solution is found to the lack of trust.
Of course, if trust is low, you might not give a true trust level. This problem can fairly easily be addressed by having a retrospective facilitator that is in no way invested in the project. The facilitator can collect the trust measurements in an anonymous manner that protects the innocent team members.
If you're finding that your retrospectives aren't providing as much value as they should be, you might want to measure the trust level. You may find that people are afraid to talk about the bigger problems.