Recently, significant progress has been made in the organic thin film transistor field, which has prompted the development and application of microelectronic circuits, organic light-emitting displays, organic bioelectronics, and organic optoelectronic detectors. However, polycrystalline or amorphous organic semiconductors are prone to many defects, and the device fabrication process introduces defects, resulting in localized trap states in the band gap to trap charge carriers. Consequently, the trap states affect the carrier transport in organic thin film transistors considerably. Thus, in this study, several methods to detect and characterize the trap density of the states in organic semiconductor devices are reviewed. Different methods utilize different approximations and assumptions, employ different measurement principles, and cover different energy ranges. Therefore, the advantages and limitations of several detection methods are compared and analyzed, and guiding opinions are presented to facilitate effective measurement of the trap states in organic thin films.