So, just to clarify, you needn't worry about vague rules - this question is relevant to my own fantasy universe. In this such universe, magic is not a separate energy, it is the ability to control energy (heat, light, electricity etc) much in the same way as the Inheritance Cycle series tackles it, if you're familiar with it. The manipulation is also pretty similar, as words and gestures must be bound to the action to cause the effect (usually), the spell is not as simple as 'fireball' either, it is an explained process - for instance: gather heat in fixed place (to create fireball), sustain flame, move flame this way (shoot the fireball). Unlike the Inheritance series however, the magic is not bound to one language, there are multiple, the words simply help structure the spell in a human brain, and having 'magic languages' separate from communicative language prevents unfortunate accidents.
While most spells will be uttered or shaped with gestures, some larger or more complex spells would take too much time to be viable in battle, so a way to speed up the process is to pre-write the spell with runes. Once the rune is inscribed, and the power source of the spell specified, the rune's spell can be activated with as little as a single word.
As I was designing a weapon for one of the main characters I realised quite an important flaw in the runes, since they are written (and probably glowing when activated as-per rule of cool), anyone who can read the language would instantly know what spell the object is capable of casting and be able to counter/negate it.
Obviously, fantasy and rule of cool dictate the runes should remain exposed, but logic would have them covered. What reason would there be to have them exposed? I wonder why this question hasn't come up in any other fantasy series.
Side note: If you could, adding how the runes are hidden would be great too, if your answer is in favour of hiding them of course.
Edit: While it isn't final, I'll add some extra info to clarify a few things - As I have it now, magic just needs to be bound to something, and you need the intent to use magic, then the spell can work. When casting in the mind, the electrical signals in your brain provide the physical binding, gestures and words otherwise provide such bindings. These things are temporary, while a rune is less so, once the spell is bound to the words made by the runes, it can be cast again by anyone who knows the activation sequence. This part could change if need be.
P.S. I'm unfamiliar with the fantasy-related tags - I usually ask about sci-fi - so if you know of other relevant tags please add them.