I don’t think you should so much notice that you’re ignorant as assume you’re ignorant. You always assign some probability to “something I haven’t thought of”. You do need to notice when you’re making an implicit assumption that you’ve thought of everything. And you need to figure out how much probability to assign to things you haven’t thought of.
I don’t think there’s any good theoretical way to figure out how likely it is that the answer is something that you haven’t thought of. You just have to practice. I’m not sure how you can practice.
I don’t think you should so much notice that you’re ignorant as assume you’re ignorant. You always assign some probability to “something I haven’t thought of”. You do need to notice when you’re making an implicit assumption that you’ve thought of everything. And you need to figure out how much probability to assign to things you haven’t thought of.
I don’t think there’s any good theoretical way to figure out how likely it is that the answer is something that you haven’t thought of. You just have to practice. I’m not sure how you can practice.