What you tell them will in part be determined by the purpose of the writing assignment. It will also be determined by what you expect students to do next something we will see in students' second question below. The best time to answer this question is while you are designing the assignment. Here are ways in which some professors deal with this question. With certain assignments for example, journals or personal responses to readings you may be looking simply to see whether or not a student has done the writing.
Some professors give bonus points for particularly full responses.
Others underscore special insights with a highlighter and put wavy lines under content that is unclear. Students value assignments that help them learn a genre they will use in their work or in advanced courses for example, a prospectus, a report of findings, or a site analysis. Since students need repeated practice to master specialized forms of writing, you frequently will be making comments to guide their revising. Sometimes it is useful to turn this question back to your students. Have them write a brief self-assessment and attach it to the draft they are giving you.