"Difficult" and "easy" are relative terms. As such, they must be evaluated in relation to something else. This isn't always obvious to people, as it is quite common to simply talk about things being "easy" or "difficult." ("It's easy to ride a bicycle." "It's difficult to get to the moon.")
The best advice I know to offer is to make the question as "tangible" as possible. Explain how you define difficulty.
Some (made up) examples, and suggestions to consider improving them:
1. Is learning X language more difficult than Y language?
First, this question is probably too broad, as in most cases, X will have some aspects that are easier than Y, and others that are more difficult--and which characteristics fall into which category likely depends on which language(s) you already know!
So for this sort of question, I suggest focusing on a very specific aspect of the language:
1. As a native speaker of W, will the grammar in X or Y be more complicated to learn?
2. As a native speaker of W, will X or Y have more new sounds that I must learn to pronounce?
Or ask a general question comparing X or Y to your native language:
3. As a native speaker of W, what aspects of X will be most challenging to master?
4. As a native speaker of W, what aspects of Y will be simplest to learn?
2. Is method X easier than Y to accomplish Z?
3. Is it easier to learn language X before Y, or Y before X?
Again, we need to know what "easy" is to you. This may be impossible to define in many cases. "Can I accomplish Z faster with X than with Y?" This may not work, as which is faster likely depends more on how much time you devote to it, than the method itself.
In these cases, perhaps a better approach is to ask about the pros and cons of X and Y, or even in which ways X may be considered easier than Y, and decide for yourself which fits your learning styles/preferences.
1. What are the benefits of X over Y?
2. How is X considered easier than Y?
3. In what ways is learning language X first beneficial to learning language Y later?