An electron moves to the right into a uniform magnetic field, as shown in the diagram. The question: What is the direction of the force on the electron? Justify your answer.
The student reasonably asks, how much justification is enough?
"The electron is moving right, so I point the fingers of my right hand to the right. The magnetic field is down the page, so I put my right palm toward the bottom of the page and curl my fingers. I extend my thumb, which now points into the page - that would be the direction of the force on a positive charge. Since this is an electron, I flip the direction of the force, so the electron is forced out of the page." -- Too much. We don't need a dissertation. On a test, one might have just a minute or so to solve this kind of problem. It's unreasonable to expect a student to provide this level of detail about such a simple problem, especially as the student becomes so proficient that this problem is easy.
"Out of the page -- first right hand rule." -- Not enough. There are three right hand rules, which are all taught differently in different physics classes. And even then, saying that a right hand rule applies isn't much beyond stating a bare answer. Of COURSE a right hand rule applies -- it's a magnetism question!
"Out of the page. Use the RHR for the force on a moving charge. Point right, curl down the page, flip the thumb because the charge is negative." -- Just right. The right hand rule in use is identified unambiguously. ("The right hand rule associated with F = qvB would have been good, too.) The mechanics of the rule are stated clearly but in a shorthand. Anyone reading this justification would recognize that this student understands why the force on this charge is out of the page.