For the development of novel and long-life polyolefin materials in the field of high-performance applications such as pipes and containers, meaningful test methods have to be used and improved. Especially in terms of environmental stress cracking (ESC) phenomena that lead to a decrease in life time of polyolefin components, the requirements of tests for an ever-increasing resistance against ESC have to be met. The full-notch creep test (FNCT) is a common method to evaluate the ESC behavior of high-density polyethylene (PE-HD) container materials. The test procedure as specified in ISO 16770 provides a comparative measure of the resistance against ESC using the time to failure of specimens mechanically loaded in a well-defined liquid environment. Since the craze-crack damage mechanism underlying the ESC process is associated with brittle failure, the occurrence of globally brittle fracture surfaces is a prerequisite to consider an FNCT measurement as representative for ESC. Therefore, an optical evaluation of FNCT fracture surfaces concerning their brittleness is essential. Due to the experimental setup, an inevitable increase of the true mechanical stress and the associated appearance of small ductile parts on fracture surfaces is induced in any case. Hence, an FNCT experiment is considered as 'valid', if the corresponding fracture surface is predominantly brittle. Based on laser scanning microscopy (LSM) height data of FNCT fracture surfaces, a universal and easy-to-use phenomenological criterion was developed to assess the validity of distinct FNCT experiments. This criterion is supposed to facilitate a quick evaluation of FNCT results in practical routine testing, which is significant for the development of meaningful test procedures within the Fourth Industrial Revolution.