This is one of those things that students get used to (as with everything - which is why we generally don't want students trying to "prepare" to do their exercises beforehand - just do the assigned quantity to the best of your current ability), because making those judgments is something you get better at as your spatial reasoning skills improve, and along with them, your ability to judge perpendicularity in 3D space.

Ultimately the way you think about it is as though you're extending the ribbon out further, as shown here, and using that structure as the basis for your arrow head (giving you a point of reference for which directions will be perpendicular to the flow of the arrow - but again, that's still going to depend on your spatial reasoning skills, which will continue to improve with practice).