In short, it’s almost impossible to judge what is ‘factual’ about Caesar, and the same can be said about anyone from that time period. We can judge whether sources were exaggerated or not based on some archaeological finds, but when all you have to go on are written sources from ~2,000 years ago it’s important to keep in mind that some of it might be flat out made up.

That being said, especially for Julius Caesar, a lot of what is written about him during the Roman period is generally considered true, for the most part, but again it is important for the reader to keep in mind the various biased that might lead to exaggeration and hyperbole in certain sources, either with the aim of ruining or praising Caesar and his reputation.

Suetonius’ Lives of the Twelve Caesars, covering Julius Caesar and the first 11 Emperors, is generally considered a mostly factual biography. Suetonius was Hadrian’s imperial archivist, so he had access to basically anything and everything that might have information on Caesar, and was also quite critical and fair in his account, discounting or discrediting certain stories that he himself doubts.

We also have Plutarch, whose biography of Caesar was written as a counterpart to Plutarch’s own Life of Alexander the Great, and was meant to be a parallel between the two men. Of course, with this comes a certain agenda within Plutarch’s biography of Caesar; while I won’t go so far as to say the story is twisted, he will have certainly chosen which parts to include and which parts to exaggerate to better portray the image that Caesar and Alexander (and by extent Romans and Greeks) weren’t all that different.

And, of course, we have the writings of Julius Caesar himself. His most famous work, Commentarii de Bello Gallico, is generally considered a factual account of his conquest of Gaul, albeit with some exaggerated numbers, but again it is important to keep in mind that the book - and the rest of Caesar’s bibliography - were probably the most biased in Caesar’s favour, being written by the man himself.

Generally speaking, we can’t really tell how much is factual and how much is mythical with any major certainty, short of a few events. For example, the story given to us by Suetonius that two beings with swords and darts set fire to Caesar’s pyre is almost certainly considered mythical (Caesar 84). Of course, that being said, there are plenty of times when we can be absolutely certain of some things. When we read that Caesar built a new harbour at Ostia, we can see that harbour there today, and know when it was built, and the Forum Julium in Rome has various inscriptions crediting the construction of the forum to Caesar, which corresponds with his biographies.

So in terms of judging the reality of Caesar’s life, our best option, ultimately, is to cross-reference between Caesar’s own writings, the biographies about him, old Roman records that might still survive, and the surviving archaeological sources. It’s all down to cross-referencing!

I hope this rambling trash sort of answers your question! I’d highly recommend reading Suetonius, Plutarch and especially Caesar’s Commentarii de Bello Gallico, they’re all rather interesting and have some great information in them.