Active open-minded thinking represents a prescriptive and normative model of good thinking and decision-making. It determines that the amount of search for information should be proportional to the importance of the question at hand, open to possibilities different from the initially preferred one, and that the confidence in the decision should reflect the amount and quality of thinking. Current measures have proven to be problematic for a deep insight into this construct. According to the need for an instrument that directly measures active open thinking as an individual characteristic, which is based on theory and represents a contextualized measure, we developed and validated a situational judgment test for measuring active open-minded thinking. Convergent validity was assessed by examining the correlation of our instrument with the Actively Opened-minded Thinking scale, and the discriminant validity was assessed through the correlation with cognitive reflexivity. We assessed the correlation of generic conspiracist ideation, misinformation susceptibility and overconfidence bias with our instrument to evaluate its criterion and incremental validity. Statistically significant correlations were obtained, but the expected level of correlation with the variables used was not achieved. As for the incremental validity, a small contribution was confirmed by explaining additional variance of misinformation susceptibility and overconfidence bias while controlling for cognitive reflexivity and the Actively Opened-minded Thinking scale. These findings suggest it is possible to create a situational judgment test for measuring active open-minded thinking, but there is plenty of room for improvement.