I find myself frequently going through "First Post" questions where the Author has made a number of assertions that are often either baseless or heavily opinionated.
I looked through our guidelines on How to Ask but the closest I could find to, If you're going to assert something in your question, prove it's real is
Have you thoroughly searched for an answer before asking your question? Sharing your research helps everyone. Tell us what you found and why it didn’t meet your needs. This demonstrates that you’ve taken the time to try to help yourself, it saves us from reiterating obvious answers, and above all, it helps you get a more specific and relevant answer!
But this does not explicitly state if you are going to assert something is happening you should prove it. Obviously in some terms they are subjective e.g. Why does X group of People support Y may not require proof. But why did X do this? Should require proof that X did Y in the first place.
Would it be helpful to add "Must Cite Assertions" to our "How to Ask"guidelines?