Transformative agreements have become an important strategy in the transition to open access, with almost 1,200 such agreements registered by 2025. Despite their prevalence, these agreements suffer from important transparency limitations, most notably article-level metadata indicating which articles are covered by these agreements. Typically, this data is available to libraries but not openly shared, making it difficult to study the impact of these agreements. In this paper, we present a novel, open, replicable method for analyzing transformative agreements using open metadata, specifically the Journal Checker tool provided by cOAlition S and OpenAlex. To demonstrate its potential, we apply our approach to a subset of publications funded by the Dutch Research Council (NWO) and its health research counterpart ZonMw. In addition, the results of this open method are compared with the actual publisher data reported to the Dutch university library consortium UKB. This validation shows that this open method accurately identified 89% of the publications covered by transformative agreements, while the 11% false positives shed an interesting light on the limitations of this method. In the absence of hard, openly available article-level data on transformative agreements, we provide researchers and institutions with a powerful tool to critically track and evaluate the impact of these agreements.