Would it be possible to approach the problem in a different way? Instead of asking questions where the user decides just yes or no, make them state their choice by affirming text.
For example, instead of:
Does a pilot's license require the applicant to know the complete rulebook?
A pilot's license requires the applicant to know
[the complete rulebook]
[the important parts of the rulebook]
Using yes or no symbols can be confusing, especially if the question includes negation (aka. "Do you really want to quit and not safe your work progress? Yes/No"). Reiterating the user's intent in a short phrase can be more understandable (i.e. "Save before quitting/Dismiss changes")
This sort of evades your question, but I think there is a good reason why there is no universally agreed upon sign for yes and no. These short binary answers always have meaning only in their context and not by themselves, thus creating a symbol for them is hard.
Considering your edited question my answer and examples are less applicable to your problem. Maybe one similar approach to solve the problem transferred to your situation would be to offer the users choices that still are more explicit than just a "yes/no" or attempted icon for those answers. For example your question "Is the sky blue?" could have two images as choices, one bright blue daylight scene and one orange-red sunset. This way you make the user repeat and affirm their choice.