The Evaluator Role
Being an evaluator is not easy. You're dealing with a delicate subject: human feelings. But done well, you can actually make the other person better. You have the power to change a person. Thus, this role should be dealt with "care" and preparation.
Two Key Ingredients
Building on strength. The best way to both start and finish an evaluation is by giving praise for the well-done things, and giving the suggestions for improvement in the middle (this is called the sandwich approach).
Giving praise and building on what the speaker did well is an essential part of an evaluation. If not given, the speaker might actually get discouraged from further speaking (especially if a new member).
Suggestion for improvement. Each speech, no matter how perfectly done, can be improved. During your evaluation, point out at least one thing, one aspect that could have been done better. A speech evaluation without some "constructive criticism" is just not a good evaluation. You might like it as the evaluator, but it does not do any good if the speaker tries to improve.
Suggestion
Below is an excellent article on how to give an excellent evaluation. It should help you.
Related
Giving Killer Evaluations, Toastmasters San Diego