In recent years, approaches to developing teacher competency in technology integration have moved away from an over emphasis on technological knowledge, to focus on the essential connections between technology, pedagogy and content knowledge (TPACK) (Chai, Koh, & Tsai, 2013a; Harris, Mishra, & Koehler, 2009; Mishra & Koehler, 2006). A number of extant studies have established the construct validity of the TPACK framework (Chai, Ng, Li, Hong, & Koh, 2013; Lin, Tsai, Chai, & Lee, 2013; Schmidt, Baran, Thompson, Mishra, Koehler, & Shin, 2009). However, it remains unclear whether the self-reported knowledge assessed by the TPACK scales, might be supplemented by preservice teachers' objective knowledge in sub-areas of TPACK. Here we use a well-established measure--discriminability indices (d') (Macmillan & Creelman, 2005)--in a novel domain (TPACK) in an attempt to evaluate the extent to which the subjective knowledge assessed by the TPACK-deep scale (Yurdakul et al., 2012) relates to preservice teachers' ability to discriminate between true and false statements about TPACK. There was only a small correlation between discriminability and TPACK-deep scores (r = 0.25, p < 0.05). The results suggest that the subjective knowledge assessed by TPACK scales could be supplemented by the inclusion of objective indices of technological pedagogical and content knowledge to form a more complete picture of preservice teachers' TPACK.