In Truman Capote's "Miriam," the clues that might show that Miriam is not real would include her characteristics and behaviors that make no sense.
Miriam's hair sounds unusual. However, if young Miriam represents the repressed psyche of Mrs. Miller, the older woman might be imagining some aspects of her younger self (long hair) with white hair (which might symbolize Mrs. Miller's advancing age).
Miriam has never been to a movie before. She says that she is not allowed in the movie alone, so she asks Mrs. Miller to buy her ticket. Is it possible that Miriam cannot buy a ticket because she does not exist? In Mrs. Miller's mind, she might believe that she takes coins from the girl, but they may be coins she took from her purse when she collected the coins for her ticket.
Mrs. Miller rummaged in her leather handbag till she collected exactly the correct change for admission.
For whose admission? When Mrs. Miller first sees Miriam, the older woman is "looking around for some distraction." Perhaps Mrs. Miller has "created" young Miriam.
When there are questions that cannot be answered by someone that does not exist, Miriam simply does not answer:
"...Your mother knows where you are, dear? I mean she does, doesn’t she?"
The little girl said nothing.
"Since I’ve waited so long, you could at least let me in," she said.
"It’s awfully late."
Miriam regarded her blankly. "What difference does that make? Let me in. It’s cold out here..."
And again, soon after:
"How did you know where I lived?"
Miriam frowned. "That’s no question at all. What’s your name? What’s mine?"
Without any complexity of character—because she is a figment of Mrs. Miller's imagination—Miriam cannot answer questions about information that pertains to a real person with a real history.
Mrs. Miller may know that Miriam is not real. After all, while she may not recognize this consciously, what adult would send a child out alone at night in the snow? Wouldn't she call the police?
"...look - if I make some nice sandwiches will you be a good child and run along home? It’s past midnight, I’m sure."
"It’s snowing," reproached Miriam. "And cold and dark."
"Well, you shouldn’t have come here to begin with," said Mrs. Miller, struggling to control her voice. "I can’t help the weather. If you want anything to eat you’ll have to promise to leave."
Capote presents a typical youngster in some ways: Miriam wants something to eat, she wants to see the bird even though it's sleeping, and she longs for sweets.
At the same time, she is more an adult version of a child, which adds to a sense of the unreal:
Mrs. Miller decided the truly distinctive feature was not her hair, but her eyes; they were hazel, steady, lacking any childlike quality whatsoever and, because of their size, seemed to consume her small face.
Miriam frightens Mrs. Miller with her child-like appearance which is in conflict with adult behaviors:
"What are you doing?" she asked.
Miriam glanced up, and in her eyes there was a look that was not ordinary.
Miriam Miller tells the child she wants her to go away, but goes out and buys things the child has asked for. Though Mrs. Miller tells Miriam to "go away and leave me alone," her behavior contradicts her words. The child arrives and Mrs. Miller opens the door:
It was not spell-like compulsion that Mrs. Miller felt, but rather, curious passivity...
Perhaps Mrs. Miller cannot say no to Miriam for she is speaking to herself—to her own psyche.