The best thing I can thing of that relates to this is based off LAIONs style attribution knn index search for their 5B image dataset. A similar approach could be done for text - search over text for similar samples. But again no guarantee however it’s fairly interpretable. the dataset of generations from chatgpt for 100M users is growing fast and searching over it is most likely improbable at the current pricing options . Also, As you said using gpt2 to measure perplexity is good for catching gpt generated text, but it’s not a perfect solution imo