In its simplest form, the ‘truth’ is not telling a lie with intent to deceive. It’s as they say in court: the truth, the whole truth and nothing but the truth. Integrity to the best of anyone’s ability.
An idealistic expectation of humans (who are, at base level, animals and selfish). But the mark of civilised society is the rule of law and justice. Hence why perjury is such a serious crime and gets punished very hard.
In many cases we simply don't know, as we were not there to witness events.
But there’s recorded history. Artefacts. First hand witness accounts. Press reports from the time. Don’t underestimate bureaucracy - and it’s historical value.
History written by victors, sure. That’s why skilled historians don’t take the victors version of events at face value. They work hard to piece together what really happened evaluating primary, secondary sources and evidence that’s available. Reading between the lines (where evidence wasn’t recorded or was destroyed etc.) with a critical eye like a detective.
As an aside, this is also why stories about the gods are useful in keeping humans on their toes a bit to behave. We might not be witness to all events but how do you know immortal gods aren’t witness?
Loads of examples in classical literature of gods being witness to wrongdoing and not forgetting. Ancient Greek proverbial expression: “The millstones of the gods grind late, but they grind fine." Gods were thought to take their time (because they have time on their side, being immortal) and wait until they had their ducks in order until they delivered justice.
Plutarch argued, "Thus, I do not see what use there is in those mills of the gods said to grind so late as to render punishment hard to be recognized, and to make wickedness fearless." But better late than never…