Well, first of all, it's clearly indicated if a Low Quality Post is an answer or a question.
I assume you're reviewing on Stack Overflow, which has an additional Triage review queue; this has the effect that all (non-audit) reviews in the Low Quality Posts queue are about answers.
What should I do in these cases?
Go back to the post and flag it as Not An Answer. If there isn't already a comment indicating that it's not an answer, you should post one (as noted by @Wrzlprmft); just copy the appropriate text from the dialog you see when you click Recommend Deletion (on a next post in the queue). Furthermore, you can hope that the other reviewers will not make the same mistake; each additional active "not an answer" flag causes the review to require one more reviewer to choose "Looks OK" to end a Low Quality Posts review.
Note that if you edit a post, that's actually a third option (but implying the post is OK) and submitting the edit will remove the post from the review queue. The post will only go through a review once, so a subsequent flag from anyone after a post has already gone through review will go to moderators. If you choose to edit from outside review, the post will go through the system's quality test again (the same one that ordinarily causes posts to automatically enter the LQ queue), and if it passes, it will be removed from the queue.