So first, I feel like this is a question akin to the question about whether Christ was a real person. I'll refer you to the FAQ for a lot of that information.

For Caesar specifically, we have several ways of saying he was real.

First, we have the archaeological evidence of his coins. These are all dated to Caesar's life and bear his name.

Second, we have the evidence of Cicero's letters. Cicero wrote about Caesar many times, and we have several letters to and from Caesar. It would mean Cicero made Caesar up if he weren't a real person.

Third, we have Caesar's own writings in the Gallic War and the Civil War. Both books claim to be written by Caesar, with a strong manuscript tradition taking them back to the 9th century AD. That's pretty much as best as we can ask for as far as ancient manuscripts go.

I'm not even going to talk about the rest of the received history of the Roman Empire; that makes no sense if Caesar wasn't a real person.