I'm an avid Toastmasters & could I suggest how we evaluate speakers and apply it to photography?
This would apply to a seminar where it's a learning experience and not competition.
What I would recommend is discovering from the maker of the print(s) what are s(he)'s objectives. Are there areas they want the evaluator to focus on? There are 12 elements that the PPA uses to determine a merit print. Does the maker want to those used as a foundation of the evaluation.
Suggest having a sheet of paper for each print to make notes. Divide the paper (could possibly be formulated in the cranium but sometimes fog gets in the way!) in thirds. The first third find some good qualities positive traits that you found in the photograph. Can't find any? Keep working on it until you do.
The second third would be one or two, maybe even three, ideas to help the maker improve next time around. Our goal should be to build a fire inside the maker so as they go out and learn from our recommendations and want to submit more.
Be careful with this part. Egos can get bruised. Our goal should be one or two and maybe three areas to go out and do better. And better the next time and every time after that.
Finally at the last third on the paper, end your critique on a positive not. Find a couple more good qualities, maybe recite again what you said during the first part and end your critique saying something like, "Good job and I look forward to reviewing the next prints you make."