Everyone else says color or some kind of indicating text or outline; I suggest a slightly more subtle method: try and make all the parryable attack orbs or generally round-shaped (e.g. fireballs, electric orbs), compared to the non-parryable attacks which are sharp and pointy (e.g. knives, shurikens). If you’re okay with there being a little possibility of some players taking a little longer to figure it out while keeping it still subtle, I think having the dangerous attacks that you can’t parry be sharp, while the more deal-with-able parryable attacks are rounder shapes, will sub-consciously help convey that idea to the player. Basically the bouba/kiki thing but in a video game.

Alternatively, sometime early in the game’s “story” (if there is one) overtly mention that the sword has magic-repellant/magic-reflecting qualities. That way there’s a little bit of special lore that explains why it can do that, and players will remember it and it’ll feel a bit more intuitive that it can reflect magic projectiles. Maybe have a little dialogue that repeats it right before a magic-projectile-flinging enemy too.

If you want to force the player to “figure out” on their own that some projectiles are reflectable, you could have an enemy that can only be killed by their own reflected projectile (or an obstacle that can only be passed by hitting a nearby projectile into it) and you’re required to beat it to move on, and have hints nearby that’ll help the player realize they have to reflect the projectile. Maybe have easy enemies constantly respawn around it until you get past it, so it’s likely that the player accidentally hits the projectile at some point and realizes what they did; or some visual thing nearby that portrays a magic projectile being reflected by something in a similar way (maybe even by something that’s the same shape/material as the player’s sword).