You have finished proofreading only when:

Question $5$ options:

you have read through the entire document without making any changes.

you have run a spellchecker and corrected all of the errors.

you have proofread the document on your computer screen and not found additional errors.

you have solicited comments from coworkers or classmates.

you are finally able to let go and stop worrying about content, typos, and format errors.