A 4-year-old child attends the immunisation clinic for catch-up vaccination. The health worker retrieves a vial of inactivated polio vaccine (IPV) that has been stored in the cold chain for 18 months. The vial label is partially illegible, and there is uncertainty about whether the vaccine has been exposed to freezing temperatures at any point. Which investigation is most appropriate to assess whether this vaccine vial should be used or discarded?