Besides Plato’s works? I mean, what if Plato made up Socrates just to use as a kind of mouth piece to explain his own beliefs and ideas? Is that not plausible? Socrates could merely be a fictional character.
It’s not as if Plato would have no reason for doing so. He could use the character of Socrates so that he wouldn’t be persecuted himself for his beliefs. I’m not saying that this is probable, I’m merely asking the question, to see if it is possible.