You can go check. The historical sites and documents are around to look at. You might have to learn several dead languages, of course. With military events the

You can go check. The historical sites and documents are around to look at. You might have to learn several dead languages, of course. With military events there were attempts to fib by exaggerating military victories, or claiming a defeat was a victory. If it's relatively modern there are usually multiple perspectives, and you can go try to find evidence at the site.