During a routine immunization audit in a primary health center in rural Rajasthan, the cold chain officer discovers that the vaccine refrigerator temperature had fluctuated between 0–10°C over the past 48 hours due to power cuts. Oral polio vaccine (OPV), diphtheria-tetanus-pertussis (DTP), and measles-rubella (MR) vaccines are stored in this unit. Which investigation is most appropriate to determine whether these vaccines can still be used safely?