Of course it is cheating. Just because you physically can do something does not mean that it is allowed. Because that's all there is to it, right? Your question is, "I can do this, therefore I think it is not cheating, is it?"
Now let's dissect your question.
By "staring", I obtain some knowledge of the questions before the paper begins (albeit only partially), which may be counted as unfair.
You should have stopped there. You are sugarcoating this. It is unfair. Have you thought about students with visual impairments? Or have you just thought about yourself?
However, this is done merely by looking at the question paper which all candidates do before the exam begins.
So? Even if the technique for cheating is simple, it's still cheating. Even if everyone is technically able to do it, it's still cheating. If I give my students an assignment to do at home, they are all technically able to go online and plagiarize something.
It seems virtually impossible to detect and in some cases is unavoidable.
Are ethics determined by whether you can get caught? Can I commit murder if I am 100% sure that I will not get caught? A surgeon can easily "slip" and kill someone, without anyone knowing better if they are good, and there is no way to avoid it. Do surgeons have a license to kill?
This is a bullshit excuse. You're better than this.
(And since I know the kind of people who frequent this website: no, I am not comparing cheating to murder. I am giving an example that proves that, obviously, "I won't get caught" does not let you off the hook, ethically.)
I have seen no academic policies regarding this; most papers I receive only instruct candidates not to open the paper before the exam begins.
It is almost certain that your university has a global policy that says cheating is forbidden. They do not need to list every single way that a student could cheat for this particular way to be forbidden. What counts is the end result. Have you purposefully obtained an unfair advantage? Yes. Therefore it's cheating. End of story.
Imagine if the policy written on the paper had been to keep your phone in your pocket, not explicitly "the use of your phone is forbidden", for some reason. Imagine you had an old-fashioned phone with a physical keyboard and a phone that vibrated in Morse code to read out SMS you receive. Do you think you could successfully argue that you have not violated the policy if you start sending questions to an outside party? After all, you have followed the policy to the letter, and everybody could technically do it...
But it's still wrong! And it falls under the global policy about cheating. What's written on the exam paper is just a reminder about a particular aspect of this global cheating policy. It doesn't mean that anything unwritten is fair game.
I've been fretting over this for days.
Perhaps this could have been an alarm signal for you. Continue to fret over this. And try to think if you are committing other unethical acts. You're a university student, not a child anymore. You're supposed to know right from wrong. Act like it, or one day, you will find yourself on the wrong end of the law. Having a "gotcha" attitude is a surefire recipe for trouble.
And for the other answer who suggested that the disciplinary board will laugh at the triviality of the offense: do you live your life only considering what you will be punished for? Shame on you.