Greater Restoration ends one occurrence of any effect, that it can cancel, on a creature you target. You do not need to know the specific effect.
Greater Restoration states that:
You imbue a creature you touch with positive energy to undo a debilitating effect. You can reduce the target's exhaustion level by one, or end one of the following effects on the target:
- One effect that charmed or petrified the target
- One curse, including the target's attunement to a cursed magic item
- Any reduction to one of the target's ability scores
- One effect reducing the target's hit point maximum
Greater Restoration does not say you need to specifically target an effect, just that you target one creature and you can then end one of those effects affecting that creature.
It also does not state that you can only end an effect that you know of, or that you must know what is effecting them. Since that the description does not specifically state that you must know what the effect is that you want to end, it would seem viable that you could end any effect that Greater Restoration is capable of ending.
Greater Restoration also says you target a creature, not an effect, meaning you don't need to know of the effect, or which specific effect it is, in order to cancel it.
For Example: Bob the Barbarian got a super cool new sword, but now he is really eager for battle and is all bloodlusty. You cast Greater Restoration on him as you think he may be cursed by the sword. Turns out the sword was not cursed, but instead another item/ability was causing this. Since that GR says it ends one of those effects, it would still successfully end the curse from a different weapon, or a charm that caused Bob to be more aggressive.
If the creature is under the effect of multiple effects, it is unclear as to which one is cancelled as the spell does not specifically state. This would be up to the DM though there are a few ways this can be handled:
- Which ever effect is the closest to what the player is trying to end. E.g. Bob is petrified and cursed by a sword. You want him to not be petrified, and to move again. The petrified is cancelled, the curse remains.
- Roll a dice, and use the result to determine which effect to end if they are affected by lots of effects and the player was not even close in what they thought was effecting the creature.
- Ask the player which effect they would like to end. (kind of meta gamey)
- End one at random and describe the effect that was ended. Also describe the effect that is still active if it is obvious/noticeable to the characters.
Keep in mind that you can cast Greater Restoration as many times as you would like assuming you have the spell slots. If you end on effect, you think/know they are under the affects of another effect, cast it again.