  Safeguarding the Intellectual Property (IP) of data has become critically
important as machine learning applications continue to proliferate, and their
success heavily relies on the quality of training data. While various
mechanisms exist to secure data during storage, transmission, and consumption,
fewer studies have been developed to detect whether they are already leaked for
model training without authorization. This issue is particularly challenging
due to the absence of information and control over the training process
conducted by potential attackers.
  In this paper, we concentrate on the domain of tabular data and introduce a
novel methodology, Local Distribution Shifting Synthesis (\textsc{LDSS}), to
detect leaked data that are used to train classification models. The core
concept behind \textsc{LDSS} involves injecting a small volume of synthetic
data--characterized by local shifts in class distribution--into the owner's
dataset. This enables the effective identification of models trained on leaked
data through model querying alone, as the synthetic data injection results in a
pronounced disparity in the predictions of models trained on leaked and
modified datasets. \textsc{LDSS} is \emph{model-oblivious} and hence compatible
with a diverse range of classification models. We have conducted extensive
experiments on seven types of classification models across five real-world
datasets. The comprehensive results affirm the reliability, robustness,
fidelity, security, and efficiency of \textsc{LDSS}. Extending \textsc{LDSS} to
regression tasks further highlights its versatility and efficacy compared with
baseline methods.
