In terms of the exam question that asked if the curve was consistent with
Ohm's Law:
I don't know if the exam asked, "How do you know?" but, if it didn't, it
missed the mark. (Of course, it might have been a multiple choice question
--eek.)
The simple addition of "How do you know?" would allow the student to
explain what the relationship should be and where the curve comes from.
My tests are half essay and half problems. Virtually every essay question
ends with "How do you know?" or "How can you tell?" Even many problems
require that they give qualifications on their answers. I'll start with
asking about something that happens in the magical world of physics and
then ask how their results might be different in the real world and how
they might take those real world considerations into account.
Few questions can be answered in a single word or phrase or number.
Another tactic I have used is to give problems that are intentionally
ambiguous, requiring the students to make reasonable assumptions, to
explain them, and to show how that limits their answers.
All of this makes the tests longer to grade, but I feel that I know what
they know better.