With my males I pay special attention to how they swim naturally just for record sake in case they come down with something later on. Some swim differently then others, it's a given. The biggest way to tell is to check what they look like at rest, at a slow swim, and when they're actually flaring at another male or their own reflection. On my boys, I've noticed that the biggest difference between engaged and defensive fin use starts with the motion of their body. If their beards are out and they're swimming rigidly with fins flared, they're defensive. If their beard is tucked away and their fins are wide, they could just be engaged or interested in what's going on.
Here's my veiltail when he's relaxed, he doesn't flair often and he's very passive around other fish.
And here is my butterfly halfmoon in his most relaxed state, second only to sleeping. He's my most aggressive male. A wellbred halfmoon male will have a caudal that expands to about 180 degrees or more (aka over-halfmoon). When relaxed, as far as my fish are concerned, they still retain that wide "fan tail" look.
They're great pics, thanks
I'm getting a new betta next weekend and my fave tail is delta. The place I'm going just has halfmoons though and I wondered if maybe they look like delta when relaxed. I like the look of their tail relaxed, so I think I'll be happy with a halfmoon. So excited