How to call a hit: don't.
See? That was easy.
There are very few universal rules of forum RP. One of them that stretches across nearly every board, from natural RP like ours to D20 combat systems is that you cannot control whether or not your attacks land or how much damage they do. Period. Don't do it. No one likes that action.
What is calling a hit? Well, as you might have gleaned from the above paragraph, calling a hit refers to the action of directly stating that your attack hits your opponent. For instance, let's say Bob and Suzy are dueling. Bob says something like this: "Bob swung his saber, striking Suzy in the face and chopping her head in half." That's calling a hit. Suzy will naturally be pissed. The act of calling a hit takes control of your opponent's character out of their hands, and violates another universal principle of RP: the only person who can determine what happens to your character is you.
Instead of saying "Bob swung his saber, striking Suzy in the face and chopping her head in half," he should have said something like "Bob swung his saber at Suzy's face, hoping to chop her face in half." Suddenly, it's an entirely different situation. Instead of directly affecting Suzy, he's merely saying what he's trying to do. He describes his action and his intent, and it's up to Suzy to determine what happens from there. If Suzy is a good duelist, she'll come up with a counter or, realizing that she can't counter, she'll take the hit.
It's remarkably simple. Properly wording a post is one of the most basic tenets of RP, and yet so many people have trouble with it. Don't say that you're striking your opponent. Don't say your shots hit your opponent. Don't. Do not. No. Nein. Nyet. That's literally all there is to it. Just by wording your posts a little bit differently, you can turn a duel from a grade A charlie foxtrot to a fun encounter with mutual give and take.
Now bear in mind, there is sort of a grey area in this whole hit calling department.
Let's presume a bomb goes off immediately between you and your opponent. It's your post, so you take damage from the blast. Your opponent has two choices (barring armor or Force or anything, this is just an example): take damage, or look like an ass. This one is kinda tricky, because you're not actually making them do anything, but you're kinda forcing their hand. Done properly, there's nothing wrong with this tactic. It's kinda sneaky and kinda meta, but so long as you don't abuse it, you're fine.
Another example of a grey area popped up in a GBA duel in the current tournament, and lost one side the duel because it's not okay. Let's say that you're fighting in a crowd. The conditions have been set so that the crowd will attack if they're provoked. The crowd is indeed provoked, and they attack your opponent. Your opponent fights their way outside of the crowd. Once they say they've made their way outside of the crowd, that's it. They've made that determination themselves, and if they don't do it right, that's on them. You can't attempt to change your position after the fact to make their job harder. If you want to duck and jive around while they're still fighting, that's one thing. But once they're out, they're out. That, of course, applies to just about any environmental element in a duel.
So there you have it folks. It really is that simple. Don't call hits. The end. If you've any questions or comments, my inbox and the comments section are open.