"We are not your enemy. It is actually OK to give us the benefit of the doubt on that sometimes."

And possibly: "If, after demanding angrily that a student come to the front of the class and explain 'what are all these dots?', said student nervously replies "um... what dots?", and you are unable to answer by simply pointing to one or two examples of the offending dots on the page of their work under inspection... Maybe it's time to get your eyes tested? Or even your sanity?"