In this study we sought to validate the discriminant ability of the Evaluation Tool of Children’s Handwriting–Manuscript in identifying children in Grades 2–3 with handwriting difficulties and to determine the percentage of change in handwriting scores that is consistently detected by occupational therapists. Thirty-four therapists judged and compared 35 pairs of handwriting samples. Receiver operating characteristic (ROC) analyses were performed to determine (1) the optimal cutoff values for word and letter legibility scores that identify children with handwriting difficulties who should be seen in rehabilitation and (2) the minimal clinically important difference (MCID) in handwriting scores. Cutoff scores of 75.0% for total word legibility and 76.0% for total letter legibility were found to provide excellent levels of accuracy. A difference of 10.0%–12.5% for total word legibility and 6.0%–7.0% for total letter legibility were found as the MCID. Study findings enable therapists to quantitatively support clinical judgment when evaluating handwriting.