I'm sure there is a valid general solution:
Consider the two sets {X, Y} and {X}. If I choose a set at random, will it contain the letter you think of?
If I cannot reference the letters, then here would be the strategy to generate a solution:
Pick two letters and find in each a property which differs it from the others.
Or,
Pick two letters that share a similarity, then find a difference between the two.
So for example, À, Á, Â can be separated with this question:
I flip a coin and choose a random property. If heads the property is "The thing on top is drawn with two lines". If tails the property is "If downwards gravity is activated, the thing on top either falls to the right to hit the ground or becomes a hat and stays on". If I flip the coin, does your letter satisfy the represented property?
Or perhaps the cheating answer:
Consider a question that can be asked such that when asked to you, you will answer "Yes" if you think of X, "No" if you think of Y, and "Maybe" if you think of Z. If I asked this question to you, what will you answer?