This weekend I was working on an article about fencing, and partially about how fencing in real life differs from fencing on page or stage. I asked if writers had any questions they particularly wanted answered, and I was surprised by how many people wanted to know how to avoid making mistakes when they wrote about it.
Ask questions. Do research.
This is, of course, as true for anything you want to write about that you don't have personal knowledge of, as it is for fencing. If you don't know, ask. And if you don't know what you don't know, say that, too.
You are a writer. It is your job to know how to tell a story, and all the bits and pieces that go with that. It is not your job to know how to parry a fleche - unless you are writing a story that features swordplay at the level where that sort of attack is likely.
The first thing to do is decide what level of expert advice your story needs. If you're writing something where the hero is being pursued through an old and creepy manor house, and he grabs a rusting decorative sword off the wall and swings about wildly with it to defend himself, well, you probably don't need that much. No one reading your work is going to expect anything beyond "the pointy end goes in the other guy." But if your hero has been a competitive fencer for most of her life, and was really good at the sport, you'll need to know not only the difference between a beat attack and a remise, but how spending time in the sport has changed her way of thinking.
Online research is a great place to start. And it may even point you in the direction of people you can ask questions of, or ask to read your manuscript and check the technical details.
And yes, Rocco Bonetti was a real fencing master. It is historically probable that he would have given fencing instruction to someone who referenced "sword" 437 times in his literary canon: William Shakespeare.