We have multiple independent eyewitness accounts for Socrates as well as contemporaneous (and unflattering) references to him from other writers during his lifetime including an SNL type parody of him in a play by Aristophanes. There is plenty of debate about the accuracy of how Socrates is presented by various writers (especially with regards to the dialogues of Plato) but his existence is virtually beyond dispute.

Aristophanes made fun of him in Clouds and Socrates even references Aristophanes and Clouds in his famous defense. He is also mentioned by Thucydides in regards to his relationship with Alcibiades. Given this solid range of mentions across ancient genres, along with Thucydides' reputation for a meticulous and factual approach (relative to his time), I would say it is more than fair to conclude that Socrates was a real historical figure and probably not far from the image that survives of him today.

Sources for this are Clouds by Aristophanes, Apology by Plato and The Peloponnesian War by Thucydides.

Probably not the right place to ask but what about Homer?