With the large-scale use of composite insulators, decay-like fracture failure has occurred multiple times, affecting the normal operation of the power system. Therefore, appropriate detection methods for this failure are quite important. This article starts with the study of the mechanism responsible for decay-like failure, summarizes the commonly used detection methods, and provides several promising detection directions. Offline approaches mainly involve SEM (scanning electron microscopy), FTIR (Fourier transform infrared spectroscopy), TGA (thermogravimetric analysis), and XPS (X-ray photoelectron spectroscopy), while on-site detection approaches mainly rely on infrared detection. However, currently there are few samples of decay-like insulators detected by infrared spectroscopy, resulting to the incomplete algorithm model. Meanwhile, infrared images quality also has high requirements for the shooting environment. Actually, joint detection is a direction to improve diagnostic accuracy. Ultraviolet (UV) pulse, as a low-cost and easy-to-use technology, has the potential to play a certain auxiliary discrimination role in infrared diagnosis. Ultrasonic guided waves can effectively observe the debonding between the core rod and the shell, which is a significant reason for the interface failure of the decay-like composite insulator; Microwaves, due to their unique properties, will bring new and reliable ways for this detection.