As I understood it, we do not know for certain, however Aristotle does mention him as his teachers teacher.

I guess his existence is heresay only one 'a' away from heresy

I guess that depends on who’s reality we are talking about. Am I in your reality or are you in mine? Perhaps we are co existing the same stream.
Maybe we are both in someone else’s stream.
Cogito ergo sum?

Socrates was real enough - his teachings and style were recognisably different from Plato's. None of Plato's (and Socrates' ) contemporaries challenged the account of his life, trial or other biographical details. You may as well ask if Plato was a literary device of Socrates.

Strictly speaking we do not... None of us were there

This article suggests that there seem to be surviving references to Socrates from at least four other comic playwrights: Eupolis, Emeipsias, Theopompus, and one who is anonymous. So Socrates was real is the consensus among scholars.

[donaldrobertson.name]

Wikipedia gives some references that seem to verify the life of Socrates. For example, Socrates was a soldier during the Peloponnesian War and was mentioned by others as fulfilling that role nobly.

How could Plato have pulled off such a prank and not been exposed?

Like any ancient character or personage, there are varying schools of thought. Some, like for jesus are easier to show that there is no evidence. For Socrates, though, there is more evidence for him than for the king of jews. That said, there are those with credible arguments against. He is mentioned in at least three plays, which again may not be real proof..............etc. Anyway, here is a pretty good summary argument for him being real.

[donaldrobertson.name]

I would never have thought of that. Thanks.

Several other people of the time like Xenophon also mention him, that's not proof but it does give you confidence.

Very interesting... if Socrates was not a real man but a literary device created by Plato, then the teachings of Socrates are the teachings of Plato. This would increase Plato influence on the world by at least a factor of Socrates...Thereby creating the teachings of Platocrates

There was a film from 1989 called Bill & Ted’s Excellent Adventure. In this film two youths traveled in a time machine and met with Socrates. He was depicted as a real man, and this is good enough proof for me.

From what I've read, I always assumed he was an actual person. I believe he's mentioned by others than Plato. Of course, that doesn't mean it's certain.

We know very little about what is actual history and what is not prior to 64 AD. Very few records still exist. Most were destroyed in the fire which burned Rome.

Was Plato a historical character or just a literary device used by Socrates?