This is a complex and interesting question because, indeed, narrative perspective can change throughout a narrative. A contemporary popular example of this is John le Carre's novels that change omniscient third person perspective with virtually each chapter. What elements indicate the narrative perspective in the introduction, the main part of the novel, and the ending?
In the introduction the basic narrative perspective is set up. The most common tools are tense and pronouns. For third person (omniscient and limited, obtrusive and unobtrusive), the verb tenses are likely to be in some form of past tense, either simple, past perfect or past progressive. An example of this is the introduction of the story (Chapter II) to Tom Jones by Henry Fielding:
In that part ... called Somersetshire, there lately lived, and perhaps lives still, a gentleman whose name was Allworthy, and who might well be called the favourite of both nature and fortune;
The past constructions here are called, lately lived, might be called. This sets the narrative in the past and away from present time. A past tense narrative may also be told from first person, so what marks them out as different narrative perspectives? The use of pronouns will be different in the two. Later in the same chapter of Tom Jones, indicative third person pronouns are employed; you'll note his, he, her:
had in his youth married a ... woman, of whom he had been extremely fond: by her he had three children, ....
Compare this to the first person narrative perspective of the introduction to David Copperfield by Charles Dickens and note the presence of the first pronoun I and the possessive my:
Whether I shall turn out to be the hero of my own life, or whether that station will be held by anybody else, these pages must show. To begin my life with the beginning of my life, I record that I was born...
If we jump to the ending and examine a story set in a frame, like Conrad’s Heart of Darkness, we'll see elements indicative of perspective in the ending. Note the quotation marks used for the speaker who is telling the story within the story that is being narrated by the frist person narrator ("I raised my head"):
"Hadn't he said he wanted only justice? But I couldn't. ... It would have been too dark ...."
Marlow ceased, and sat apart, .... Nobody moved for a time. "We have lost the first of the ebb," said the Director, suddenly. I raised my head.
So an element that indicates perspective in the ending in the case of a frame story is quotation marks of direct dialogue (in this example the dialogue happens to be the entire story and contains its own internal dialogue). You'll also note tense and pronouns are continuing elements.
In the main part of a narrative, there is more opportunity for complexity but the elements of tense, pronouns and quotation marks of direct dialogue remain constant indicative elements. At times, a narrative may change from third to first person if the narrator relates a portion of the story through the words of a first person participant. In that case, the embedded first person narrative would be set off by an extended quotation as in Heart of Darkness or by a narratorial indicator something like: “Then she said to me …” or “As he said in his own words ….” A first person narrator can do the same by saying something like: “I'll let her tell it in her own words.” These are some elements that indicate narrative perspective throughout a narrative.