If someone who doesn't know your background says that you sound like a native speaker, it means they probably noticed something about your speaking that made them realize you weren't a native speaker
Ikiwa mtu ambaye hajui historia yako anasema unasikika kama mzungumzaji wa asili, inamaanisha kwamba labda aligundua jambo fulani kuhusu uzungumzaji wako ambalo lilimfanya atambue kuwa wewe si mzungumzaji asilia.