Before I answer, I would ask you as DM; What difference does it make?
What would you change in your game if you knew the players knew Princess Q was a spy? Would PQ change tactics? Would PQ know that players know it? How did she find out because nobody says anything uncharacteristic on the outside?
To me it sounds like you really want to know if your DM skills are interpreted correctly by the players. You say you dropped clues. But since no one says "she's a spy," you're curious to see if you're playing your NPCs properly.
I could be completely wrong, but that's what I think about first.
With that in mind, it's best to ask players about ALL NPCs in general.
Don't say, "So what do you think of NPC X?" Ask for more global feedback, such as, "I tried adding a background story to some of the people you meet so they are not so two-dimensional. Does that matter?"
If they can't say it, it's time to find a new way to find out information. Maybe they're not looking subtly. Maybe they're not invested enough in the story. Maybe they see the clues, but think they are pointing to someone else.
If they say they can say it, ask for examples. Someone will likely report for PQ and you can go from there.
I ask for this kind of feedback all the time. In a session I held a few months ago, I gave them a few notes / sketches from the local scholar about how to solve the puzzles in the dungeon they wanted to explore. I made some clear, some more subtle, and a few unrelated drawings. After it was all over, I asked how and if the notes had helped. Most were found out, some made no sense until I told them, and even some of the "wrong" notes became part of the adventure as they tried new things.
Ultimately, this feedback helps me shape future meetings because I know what they're paying attention to.
Just ask for feedback about the adventure, especially about the NPCs, and see what they have to say.