The main goal if I understand is that student actually understood what exercise asked him to do. Typically, if a student did not understand what exercise is about - it will clearly be visible. There is a section of "mistakes", which are good indicators that a student requires a little more pages. Generally, I'd first look at mistakes section and seek for some. After that I'd compare the actual work with example homework to spot any things that'd indicate the lack of understanding of the purpose of the exercise. Or the lack of actual effort (scribbling). But this is just my opinion, I didn't do any critiques on the community platform yet! Derived it from common sense and what Uncomfortable says in lesson zero about that student shouldn't get a perfect work done, but should complete it with a general understanding of what is going on and move forward with these concepts in mind.

To add to what f8 said, I recommend to start critiquing after you get a lesson marked as complete by other person on the community platform. Here's a guide thingy I made for critiquing lesson 1, check it out if you are unsure how to go with it Lesson 1: https://pastebin.com/dYnFt9PQ

You can also look at how other people critiqued submission, ideally TAs for l1 and boxes, and uncomfortable with higher ones

To add to what the others have said, it might also be helpful to take a look at what the purpose of each exercise is about. For instance, in Rotated Boxes, the main purpose is for students to understand how forms are supposed to fit in 3D space, and to use neighboring objects as guides for how you arrange them in it. When reading through the common mistakes, try to see how avoiding them can contribute to a student's understanding of the purpose for that exercise.

As an example, two common mistakes in the Rotated Boxes exercise are to not actually rotate your boxes but instead have adjacent boxes converge to the same vanishing point, and to have inconsistent widths between boxes. If a student has too many inconsistent widths, then it's a sign that they may not be considering neighboring box edges as guidelines for how close you place those boxes.

Also, if a student has too many boxes that are not rotating, then it's possible that they don't understand the idea behind a box rotating in 3D space. It's not that you necessarily have to get your perspective lines to be absolutely perfect, but to at least understand the idea behind it and use neighboring boxes as hints (e.g. edges of boxes further to the sides taper more than that of adjacent box edges closer to the center in that direction).