The friend who helps you study for a test they aren’t even taking is in it for you, not the grade.

Help with no upside in it is the clearest signal there is. Studying for somebody else’s test means spending an evening on material that cannot benefit them at all, which strips out every other motive and leaves one: they want you to do well. That kind of evidence is rare and easy to miss, because it tends to arrive casually.

Name it out loud to whoever has done that for you, since people are rarely told. Then look for your chance to do the version with nothing in it for you: the lift, the practice partner, an hour of quizzing somebody on a subject you will never take.