Something as simple as a reddish aura or a unique sound to these types of attacks would save players the time figuring out if the attack is failing the parry because their timing is off or because it actually can’t be parried.
Ghost of Tsushima like?
But it’s fun to discover what can and cannot be parried … That to say - almost everything is parry-able in the game (including sirens, wolves, their mutations …), so you can assume up front that everything that isn’t a clean charge, grab or a long special attack can be parried. There are also a bunch of potential bugs.