The remarkable capabilities of large language models stem from massive internet-scraped training datasets, often obtained without respecting data owners' intellectual property rights. Dataset Inference (DI) enables data owners to verify unauthorized data use by identifying whether a suspect dataset was used for training. However, current DI methods require private held-out data with a distribution that closely matches the compromised dataset. Such held-out data are rarely available in practice, severely limiting the applicability of DI. In this work, we address this challenge by synthetically generating the required validation set through two key contributions: (1) creating high-quality, diverse synthetic data via a data generator trained on a carefully designed suffix-based completion task, and (2) bridging likelihood gaps between real and synthetic data, which is realized through post-hoc calibration. Extensive experiments on diverse text datasets show that using our generated data as a held-out set enables DI to detect the original training sets with high confidence, while maintaining a low false positive rate. This result empowers copyright owners to make legitimate claims on data usage and demonstrates our method’s reliability for real-world litigations.