Research on the psychology of conspiracy theories has shown recent steps towards a standardization of measures. The present article seeks to continue that trend by presenting the Flexible Inventory of Conspiracy Suspicions (FICS), a questionnaire template that can be adapted to measure suspicions of a conspiracy around nearly any topic of public interest. Compared to conspiracy belief measures that ask about specific theories on a given topic, the FICS is worded in such a way as to provide relatively stable validity across time and cultural context. Using a hybrid approach incorporating classical test theory and Rasch scaling, three questionnaire studies on Mechanical Turk demonstrate the validity of the FICS in measuring conspiracy suspicions regarding 9/11, vaccine safety, and US elections, with good psychometric properties in most situations. However, the utility of the FICS is limited in the case of climate change due to the existence of two opposing conspiracy theories that share essentially no common assumptions ('climate change is a hoax' vs. 'there is a conspiracy to make people believe that climate change is a hoax'). The results indicate that the FICS is a reliable and valid measure of conspiracy suspicions within certain parameters, and suggest a three-level model that differentiates general conspiracist ideation, relatively vague conspiracy suspicions, and relatively specific conspiracy beliefs.