How does Marcus Aurelius distinguish real goods from what most people think is good?

Marcus does not argue the point in the abstract. He gives a test you can run on yourself. Notice what you are willing to hear said about the things you call good.

If you truly hold prudence, temperance, justice, and fortitude to be good, he writes, then having conceived them as such, you would not endure to listen to anything which should not be in harmony with what is really good (Meditations 5.12). A joke that mocked justice, or a clever line that made courage look ridiculous, would simply offend you. It would not land, because it contradicts what you actually believe is good.

But run the same test on wealth or fame. Marcus points to a line from a comic writer, one so coarse that repeating it about virtue would be unthinkable, and observes that people happily receive it when it is said of wealth and of the means which further luxury and fame, as said fitly and wittily (Meditations 5.12). The joke works. It lands as clever rather than offensive. And that reaction gives you away. If a crude joke about wealth strikes you as funny rather than false, some part of you already suspects wealth is not the kind of thing that deserves reverence. As Marcus puts it, even the many perceive the difference, whether they admit it or not (Meditations 5.12).

This is the popular misreading worth naming directly. Stoicism is sometimes taken to mean that money, health, and reputation do not matter at all, that a Stoic should feel nothing about losing them. That is not what Marcus is doing here. He is not telling you to stop wanting a comfortable life. He is asking you to notice that you do not actually believe comfort is good in the strict sense, the sense that would make it a component of a well-lived life. You believe it is useful, or pleasant, or worth pursuing when it costs you nothing else. But you do not believe it makes you a good person, and the comic writer's joke proves you know this, because you would never let the same joke touch justice or courage.

The deeper reason shows up elsewhere in the same book. Marcus reasons that death, life, honor, dishonor, pain, and pleasure all happen equally to good people and bad people, since none of these make a person better or worse (Meditations 2.11, related passage). If something falls on the just and the unjust alike, without ever affecting which one they are, then it cannot be the thing that makes a life good or bad. Wealth arrives for cruel men and kind men both. Justice, prudence, courage, and temperance do not: they are qualities of the person who has them, not events that happen to a person regardless of character. That is the line Marcus is drawing. Real goods change what you are. Popular goods only change what you have.

What changes for someone who takes this seriously is not their circumstances but their attention. You stop treating a raise, a promotion, or public praise as evidence that your life has gone well, and you start asking a different question: was that choice prudent, was it just, did it show restraint or courage. The externals can still be preferred. Marcus is not asking you to refuse comfort. He is asking you to stop confusing comfort with the thing that actually makes a life worth living.

The practical version of his test is available anytime. Notice the next time a joke about money, status, or luxury strikes you as merely witty rather than as an attack on something sacred. That reaction is data. It tells you what you actually rank as good, whatever you might say if someone asked you directly.