Working out whether someone's teasing or being unkind is genuinely hard — the exact same words can mean completely different things depending on who says them, how, and why. This isn't about getting it "right" every time. It's about learning what to look for, and knowing it's always okay to just ask.
Nobody gets this right 100% of the time — not kids, not adults, not anyone. Tone, faces, and relationships all carry meaning that words alone don't, and reading all of that at once is a genuinely difficult skill. There's no shame in getting it wrong, and asking "wait, are you joking?" is always a completely reasonable thing to do.
The other side of this: saying "I'm just joking" after upsetting someone doesn't automatically undo the upset. If someone tells you a joke hurt them, the kind response is to say sorry and stop — not to insist they should have known it was a joke. Meaning something as a joke and it landing as one are two different things, and the second one is what actually matters to the other person.