Readers asked to critique a translation often seem paralyzed with doubt. They wonder what sorts of critiques they can possibly make without being able to read the source text. What if they suggest a change that isn’t faithful to the original?
My advice is not to worry about such things. Try to forget that the text is a translation; just critique it like you would an original work in English, and let me worry about how to address the problems within the constraints of the source text. I’m trying to create something that (a) “works” in English and (b) is faithful to the source. I can handle part (b) on my own; where I need your help is part (a).
So what should you look for? Anything that seems odd, weak, or lacking. That means typos, poor grammar, and awkward wording, certainly. But don’t hesitate to address deeper issues, too: unconvincing dialogue, mismatched register, jokes that fall flat, insufficient motivation, etc. And remember: even if you point out something that can’t be altered, you’re letting me know that a problem exists, and perhaps providing invaluable insight on how to mitigate it.