This paper describes the design of a prototype transparency tool for communities, public and private organisations, and other stakeholders to use when developing, deploying and interrogating public space Internet of Things implementations as part of wider smart city initiatives. These deployments need to be accompanied by effective public communications and policy, and consideration of factors such as data privacy, so as not to negatively impact citizens. The intention of the tool is to guide transparency of these deployments in order to support trusted IoT ecosystems. The tool, which was developed as both a physical card deck and a digital implementation, was developed through a combination of participatory work and other research to understand the information requirements of citizens and other stakeholders. We suggest that further development of such tools can effectively support trustworthy IoT in smart cities.