Grading and giving feedback to student submissions automatically is becoming more and more necessary with an increasing amount of students. To verify the correctness of student-written definitional interpreters, a program equivalence approach has been implemented, improved, and extended with new rules to make it more suited specifically for verifying interpreters. This approach is able to soundly recognise two different interpreters as equivalent. Interpreters can thus be compared to a correct interpreter to verify their correctness or be grouped with equivalent interpreters to be graded in batches. Using program equivalence in combination with other verification approaches can improve the process of giving feedback to students and help build up a collection of common errors made by students.