We aimed to develop a tool to identify members of the public health workforce and classify them using categories developed for the Chief Medical Officer's project to strengthen the public health function. The tool was developed to gain a picture of London's public health workforce, and needed to be reliable and easy to use in many settings inside and outside the health service. We needed to be able to classify posts from brief information without interrogation of postholders, so that the entire workforces of large organisations could be classified from information provided by only a few key informants. Key questions and decision rules were defined by presenting interviewees in public health with brief information on nine jobs and discussing with them the process by which they determined whether each post was in the public health workforce, and if so, in which category. The questions and decision rules were refined into a classification tool which was presented as a flow diagram and a questionnaire. Application of the tool revealed that it was understood by key informants and resulted in classifications which were accepted by the researchers. The tool has now been applied extensively in London and yielded useful results. Many other applications in public health workforce planning and development are anticipated.