If they can play from beginning to end. If they've practiced, you can tell.
If the piece focuses on something specific -- loud/soft, staccao -- then they better play that, or then I would send it back another week.
If the student is getting bored, move on after awhile, even if they can't play it well. Just tag it to work on in the future.
And depending on the student, some are achieving more, practicing more, able to do more, etc. than I would expect more.
If they make a dumb mistake, I wouldn't focus on it. If it's an issue where they always make fluky mistakes, then I might -- they need to learn to weed those out.