The Scenario
I am the DM for a group who have just entered a town that was attacked by cloud giants a few days before their arrival. The players and characters don't know this though, so part of the mystery is trying to work out what happened.
One of the characters is a Ranger with giants as their Favored Enemy. The Favored Enemy feature says:
You have advantage on Wisdom (Survival) checks to track your favored enemies, as well as on Intelligence checks to recall information about them.
As an example, one of the major clues is the fact there are dozens of boulders scattered around the town that were dropped from a cloud giant castle from a great height. As well as footprints that were left behind when they eventually descended from said castle.
The Problem
Although the Ranger has giants as their favored enemy, it isn't immediately apparent that giants are at fault here (the town has been abandoned so there is no one to ask about what happened). However if the player asks to take a look at the boulders or tracks left behind, I would have to ask them to roll the check with advantage. This will immediately tip the player off that their Favored Enemy ability is coming into play as for anything else, that check would be made as a straight roll.
Obviously if the player succeeds on the check, it won't be a problem, as they would have learned giants were here. But a failure on the check would provide no in-game knowledge for the character, but still float meta-game knowledge in the air for the players.
The Question
Before a Ranger knows what they're looking at, how do I allow a player to use their Favored Enemy ability without providing spoilers on a failed check?
Best Answer
You make the advantage roll in secret
In situations where I don't want players to know they have advantage/disadvantage for any reason I simply roll a d20 for them myself and work out the results. This usually works out just fine because the players do not know if I am simply rolling an opposed check, for example.
I also make it a habit to roll d20s behind the screen for no reason which also helps with the ruse.
This does require that you either know their modifier or keep an eye on what their d20 result was and calculate it from there.
There is, however, one issue I have found with this method: it messes with abilities that allow players to reroll before knowing the results and things like the Lucky feat so it is not an universally applicable solution.