Self-directed learning is an important form of adult learning (Caffarella, 1993; Knowles, 1975; Knowles, Holton & Swanson, 2005; Merriam, 2001; Merriam & Caffarella, 1999). The strategies of self-directed learning allow adult learners to cope better with their studies while fulfilling family, work and other commitments. This study conceptualised and validated a learning diagnostic test in the context of SIM University (UniSIM), an adult learning institution, in Singapore. The learning diagnostic tool allows the students to identify their strengths and weaknesses in areas of self-directed learning. The study employed a survey research methodology. 1,960 students responded to the survey, and the statistical methods of exploratory and confirmatory factor analysis were used in the analysis. The confirmatory factor analysis showed that the final model of self-directed learning, as actualised through the items of the learning diagnostic tool, has a good fit on the data, thus affirming the strong validity and reliability of the tool. This validated learning diagnostic tool currently serves as one of the student learning support measures to help UniSIM students to be self-directed learners.