A friend of mine is trying her hand at DMing for the first time using the Beginner's Box. I'll be one of the players, so I don't know the contents of the adventure; however, she seeked my advice regarding an interesting problem.
From what I understand, somewhere in the adventure there is a magic fountain whose water gives a random bonus/penalty to those who drink it (to be rolled on a table). She asked me if the drinker is aware of the bonus (which can manifests itself later, like "+2 on this skill check, once"), if he isn't at all (and so has to connect the bonus to drinking the water) or if he becomes aware it was the magic water's effect as soon as the bonus manifests itself.
My take as a DM is that if the bonus gives a physical or mental, immediate, bonus/penalty (like "your DEX decreases by 2 for one hour") then the character should be aware of the sudden clumsiness or whatever other effect the water has on him/her.
A variant could be that drinking the water gives either that or, if the bonus/penalty is more tied to a game mechanism ("+4 to hit and damage for the next 3 attacks") then the character could feel a positive (or negative) "tingle" when he drinks the water - so he can connect the two things by itself.
As a more experienced DM I also warned her how NO CHARACTER will ever just "drink the water". They most probably will "wet a finger and lick it to see what happens!" or sprout inanities like "I drink it, but it isn't that I ACTUALLY drink it... I drink A BIT of it.... and it if is feels good, THEN I drink it...!" - and all sort of pitiable attempts at "Opening the door, but without really opening it, ya know?" that make a DM life so interesting.
Anyway, what would be your take on it? My suggestion was to make her call as a judge according to what happens and move on (*), but since, as I said, this is her first whirl at the art of DMing she is still in the "anal" phase (I guess that, by the second game, she will have got over it, as everybody does). And, BTW, it is an interesting question.
(*) As a DM since 28 years, should the characters attempt the "I lick a wet finger!" stunt I would make a water elemental appear, have it shaking its head while covering its face in incurable depression, and then have both it and the water's magical power just disappear. But that's me ^^
There's no real official rule. It depends on how much bookeeping you want to do. It's obvious SOMEthing happens to a drinker...
"A character who drinks the water while it’s still magical glows with a golden aura for a few seconds..."
If it was me I'd just tell them what the bonus is so the player can track it!
Yeah, this is the sort of thing left up to the GM to decide how much work they want to do.
Generally, I will inform the player what happened so they can do the math. If it is a group overly prone to meta-gaming, I might choose not to tell, but most of the time I do.
As for the finger licking tactic, I would apply any penalties instantly. If it was a bonus, they wouldn't get it unless they actually drank it. I might even have positive magic fade and not affect them if they tried to "test the waters" first so to speak. As in the magic affects them on their first contact with it, but if they don't drink enough they don't get the benefit, while a negative effect will occur regardless of how much they "consume".