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.
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.
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?