The Open Charge Point Protocol (OCPP) has been considered the de-facto standard for communication between charge points and the central management system of the charge points. This paper presents a novel messages testing tool based on OCPP 1.6 to evaluate the conformance of the central system to the field category definitions specified by OCPP. The authors propose the test method, discuss the test case scenarios (positive and negative) used by the messages testing tool, and describe how to set up a test case to examine the system under test. Additionally, this study highlights the difference between the messages testing tool and the OCPP 1.6 Compliance Testing Tool (OCTT) provided by the Open Charge Alliance (OCA). To test its reliability, the message testing tool has been applied to an open-source central system platform. The result shows that the system under test is able to pass 100% of the positive test case scenarios, but only 33% of the negative test case scenarios. It is worth noting that similar results may occur in other central system platforms as well. The study's findings underscore the significance of using comprehensive messages testing tools during the development and release of central system platforms. This ensures conformance to the OCPP field category definitions and promotes reliable interoperability between charge points and central management systems.