"Other than ruins and artifacts" is a nice way of dismissing everything in the archaeological record.

We have primary source texts, coinage, stelae, statuary, inscriptions, battlefields, tombs, structures, sunken ships, abandoned cities, and garbage dumps.

What is it, exactly, that you are doubting the historical understanding of? There's plenty we don't know and a lot of what we think we know is based on educated guesses. Being specific would allow someone to address the specific uncertainties and gray areas of whatever you're interested in.

Well, we have contemporary literary sources such as Plutarch, Virgil, Cicero, and others. Seeing as their descriptions tend to match up and coincide with physical archaeological evidence it's pretty hard to doubt the authenticity of what happened (at least to a degree).

There is certainly some room for nuance here. The Romans were a literate people from early on, kept records obsessively, and had their writings held in great esteem by subsequent western cultures. Because of these factors, there is a lot of primary source material that still exists. A good deal of what is known about the fellow we know as Julius Caesar was even written in his own hand, his Commentaries on the Gallic Wars.

That being said, what we would now consider "history", as in a completely factual record of past events, was something of a newer thing in Roman times. Histories in the ancient world were often written to prove a point. A record of a battle that claims the king slew 100,000 enemies was intended to convey how powerful that king was. Divine influence often featured heavily into accounts, and it can be hard to tell where myth ends and history begins. The Greek historian Thucydides wrote a History of the Peloponnesian War in the 5th century BC that was among the first works of what we would consider history today.

By the Augustan period of Rome, history was mostly just that but not always. Tacitus wrote in the late 1st century and devoted portions of his works Agricola and Germania to a sort of lecture on how corrupt Romans were compared to the noble Britons and Germans. But the parts that are history were accurate as far as Tacitus' sources could take him.

One other limitation of ancient historical writings is that we tend to think of, for instance, the Romans as all having lived at about the same time, which for us was "a long time ago". There is a famous work by Suetonius called The Lives of the Twelve Caesars, covering the Roman rulers from Julius Caesar to Nerva. Suetonius wrote this book about 150 years after the assassination of Julius Caesar, which is the same gap of time as a modern historian writing about Abraham Lincoln. Cassius Dio's history of Rome was written in around the year 220 and recounted events in the Roman Republic that were as old to him as Shakespeare is to us.

However, two answer your question, the lives of Julius Caesar and Augustus are very well documented by contemporary sources that were much more interested in fact than myth. We know more about them than many other much more recent historical figures thanks to the large number of surviving Latin sources.