It feels like WWCYO is still a good question, abstractly, to ask though even in cases where you don't think there's a good contrary argument, or don't have the knowledge, or it feels more value based.

For example, if I apply your example question to my beliefs, I don't necessarily come up with any specific answer, because I'm not super well versed in philosophy. It highlights that my belief that I exist might be on shaky ground, or it just might not be testable at all, but I'm open to being convinced otherwise.

And so that's the more abstract answer, which is : if I saw an argument that seemed rational to me, I might be convinced that I don't actually exist, but I'm sticking with what I believe for now. That's all you need -- WWCYO doesn't mean that there is a valid contrary argument, but that you're open to hearing one and changing your views. If the belief is more about experience, of course, you can get more specific about your null hypothesis.