While the study of threshold concepts is a growing area of research, their identification has not proven to be an easy process. However, identification matters because of the potential impact of threshold concepts on the learning experiences of students. A dialogue amongst lecturers and/or students is common to the literature on identification of threshold concepts. This dialogue, with the inclusion of educational developers, has been called "transactional curriculum inquiry" (Cousin in "Researching learning in higher education," Routledge, New York, 2009). Diverse methods across a variety of disciplines have explored the identification of threshold concepts, including semi-structured interviews, analysis of exam responses and observation of classroom behaviour. A selection of these methods and disciplines is discussed in order to highlight two main challenges inherent in the identification process: first, the involvement of the wider professional and/or public community, and second, a lack of agreement amongst research participants about the threshold concepts within disciplines. This paper proposes that the transactional curriculum inquiry process should be extended to involve parties beyond the educational realm (e.g. the professional community) and that the use of consensus methodology offers the potential to facilitate agreement across the transactional process.