In fanfiction it's customary to give the POV person in an author's note. I don't see the need for such a note. It's never done like that in published books. Regardless, I always know who the POV person is.
Why has that unnecessary author's note become so common in fanfiction?
When I read my first fanfics about ten years ago, the stories were not riddled with such notes. There were quite popular writers, though, who informed the reader with the POV note. It's my assumption that many less popular writers followed their lead, and nowadays it's part of the fanfiction style of writing.
Of course it's possible to tell the POV to the reader in the story. In Pride and Prejudice Jane Austen begins a chapter like this:
"Elizabeth, as they drove along, watched for the first appearance of Pemperley Woods with some perturbation; and when at long they turned in at the lodge, her spirits were in a high flutter."
There is no need to put an author's note with that. From the use of the third person singular, it's obvious that the POV person is the narrator and, from the content of the sentence, that the narrator's attention is on Elizabeth. This is an example of the third person omniscient perspective which is the most common in all literature. The omniscient narrator knows everything about the story and can tell about anything and anybody.
Often the narrator is restricted in one way or another. A limited narrator perspective means that the narrator is one of the characters, usually the main character. He or she has limited knowledge about the story as a whole. For example, such a narrator cannot tell what another character thinks (unless the narrator is a telepath) or what happens outside his sight. A subjective narrator is a combination of limited narrators: POV jumps from one head to another. All the narrator types described above are commonly used with the third person singular.
What about other persons? The first person singular is a common POV in fanfiction, too. How does one avoid author's note in this case? It's a bit more tricky than with the third person singular, but it's not impossible. In
Challenges Ela begins the story like this:
"I was lounging on my couch enjoying the sensual sound of the purring, seductive voice. I knew what the man was saying: I had heard his voice-mail several times. He had the kind of voice that went straight to my cock. Soft. Melodious. Full. Dark. He had me hooked.
What he was saying was as seductive as his voice.
Dear Mr. B,"
The first person singular is obvious from the use of I, my and me. At the end of the excerpt the name of the narrator, Mr. B., is revealed in the voice-mail he is listening. The first person perspective is always limited or subjective (in the case of alternating person views), never omniscient.