It would be hard (if not technically impossible) to prove it wasn’t trained on specific material. To prove it was, however, it’d be sufficient to show that the

It would be hard (if not technically impossible) to prove it wasn’t trained on specific material. To prove it was, however, it’d be sufficient to show that the model can be made to produce training data ‘verbatim’. In this case, perhaps a certain sentence would be read in a way that is so similar to some piece of source material so as to make it very unlikely to not be the direct result of training on that data.