Pedagogical content knowledge has been a useful construct for conceptualizing the knowledge-base that supports reform teaching practices. In physics education, we are still far from having established methods and instruments for assessing such knowledge. In an attempt to begin exploring possibilities for assessing instructor knowledge of student thinking, we asked a small sample of college physics instructors to take the Force Concept Inventory in two novel ways. Instructors were first asked to indicate the answer they think a typical novice student would choose prior to instruction and then to estimate the fraction of students answering correctly after instruction. We analyze how instructor responses compare with actual student data at our institution and discuss questions with significant mismatch-either ones that instructors overwhelming succeeded (or failed) at identifying student difficulties or questions where instructors were overwhelmingly pessimistic (or optimistic) about student performance. Implications for future work in this area of assessment are discussed.