<p>How can I ask a teacher if he/she is going to write a good rec or an average one without coming off rude?</p>
<p>"Would you be willing to write a strong recommendation for me?"</p>
<p>You will probably get strong recs from teachers you know well and speak to outside of class. If you know the teacher on a personal level (aim for that kind of a relationship), then it should not be strange to ask an honest question and to expect an honest answer.</p>