When players Discern Realities, I have a tendency to think that their questions will imply relevance. For example, if a players asks “what here is not what it appears to be?”, then I take it that there must be something that isn’t what it appears to be.
The reason is that I feel that I cheat the players by answering in the negative. “Nothing” is a boring and dissatisfying answer. Giving answers they already know is also pretty dull, and it doesn’t add to the story.
The hardest question to answer, I think, is often “What should I be on the lookout for?”, at least without telling them something obvious they knew beforehand.
Have other people had problems improvising when a player Discerns Realities?