This study outlines the development of the Design Log Instrument (DLI), which is intended for use in identifying moments of abstraction as evidence of STEM content knowledge transfer. The DLI prompts participants to be reflective during technological/engineering design challenges. During the development of this instrument, a three-phase, multiple-case, embedded design was used. Three distinct phases accommodated the collection and analysis of data necessary for this investigation: (1) pilot case study, (2) establishing content validity, and (3) establishing construct validity. During Phase 3, data from the DLI were collected at each of seven work sessions from two undergraduate design teams working through different engineering problems. At the end of Phase 3, a comparison of abstractions found in DLI responses and observation data (audio/video transcripts) indicated the extent to which the DLI independently reflected the abstractions revealed in observations (audio/video transcripts). The results of this comparison showed that the DLI has the potential to be 68% reliable in revealing abstracted knowledge.