A screening test will evaluate the likelihood of the presence of dyslexia. Opportunity Knocks uses GL Assessment's Dyslexia Portfolio for this purpose, a tool explicitly designed to identify children who are experiencing difficulties known to be associated with dyslexia.

The Portfolio measures literacy, memory and processing skills. It is administered together with tests of general cognitive ability, in the form of Raven's Standard Progressive Matrices Plus and the Mill Hill Vocabulary Scale. The Matrices assessment tool is a nonverbal measure of cognitive ability, while the Vocabulary Scale measures verbal knowledge.

Testing takes approximately two hours and yields a 'discrepancy index' score, ranging from 'no signs of dyslexia' to 'severe dyslexia'. The index score is calculated by comparing results to those obtained from students with an existing diagnosis of dyslexia. The summary report also highlights any strengths and weaknesses, important for developing a learning plan.
A screening test is often sufficient on its own to identify reasons for difficulty with reading, however it can be followed by a full assessment if that seems warranted. Generally it is recommended that the student first undergo a period of targeted intervention to see if performance improves.