Perhaps something understated like a colored gem (perhaps it pulses slightly, representing the life-force / attachment put into it by the creator of the materia) near the grip of the weapon, or somewhere appropriate on armor?

It reminds me a bit of the colored gems on the sylphs, which give a clue as to the elemental affinity for that particular sylph.

Something similar here would be an in-game visual clue as to the materia-type slotted into item.