I think you need to realise that there is a fundamental reason why it is hard. Films, of the Hollywood type, are made for two intertwined purposes: entertainment and profit. Actually the former is subservient to the latter, so a film that is unentertaining but profitable is also quite likely, but generally a high level of entertainment serves to drive profit, that's the functional value that drives the film industry.

Of course, this is not set in stone. Documentary films are not driven by quite the same factors, but we're not talking about them at present.

Historical accuracy is secondary to the goal of entertainment, and will almost always be sacrificed if it is considered detrimental or even contradictory to the telling of a compelling, entertaining, and most of all ticket-selling story.

Again, this isn't a universal law. One can dream of a utopia of well-historically-educated citizens that pilloried films with historical inaccuracies and lauded those with historical precision, but that's not this current world.

Hence, you are likely to always be frustrated with the historical accuracy of films, because their purpose is not historical accuracy.

You can always check what's been pilloried at /r/badhistory, for what to avoid, but they don't get everything. It certainly told me to avoid shows like Vikings.

As talondearg said, they don't make them to give you accurate history. Anything gets sacrificed to a better storyline.

You might look for sites that specialize in an era that do film reviews. One site dedicated to Rourke's Drift told me everything wrong with Zulu, historically. Halsall has an ORB site with a list of movies, often noting how badly off-kilter they are, or the ones that get it pretty right. Like Best Medieval Films by historical accuracy, which is at least positive input.