Is it a magazine story or a picture book? How do you know? Consider these factors.
How does the story read aloud? Can you see a kid wanting the story read over and over and over? Will an adult be willing to do that reading? This is about the language of the story. It's not whether it is written in verse, but whether the language is fun to read aloud. Often phrases will be memorable. There may be a rhythm. There may be repetitive language, almost like a chorus in a song, although much shorter. Does the language itself add to the story? Do words roll off of the tongue or are they difficult to read aloud?
Next consider whether page turns are going to be important in building the tension or the humor of the story. With magazine stories page turn doesn't usually hold any significance. Picture books are totally different. Page turns can enhance the drama, create an expected pattern, affect the pace of the story. Page turns can set up a surprise or twist. This is where creating a dummy is helpful.
Then think about the illustratability of the story. Will one image suffice or will it need many images to complete the story? Will the reader get all they need from the words? Or will art work fill in details the words leave out? Magazine stories often have a fair amount of description. Picture books don't. Use a dummy to check for illustration possibilities.
How to Make a Picture Book Dummy
Making a picture book dummy is helpful when looking at both page turns and where the story is illustratable. Print out your text and cut it up where you think the natural page breaks would be for a 32 page picture book. Take 8 sheets of paper and fold in half like a book. The story can start on page 3, 4 or 5. As you lay out the words can you envision changing images for each spread? Or are the characters static? Can you see more story being told through the pictures? If you can't see different active images for each spread and the story being enhanced by those images, you probably have a magazine story.
Now look at pacing. Is there anticipation as you turn the pages? If you break lines in different places, can you change the pace? Does a page turn create a surprise or an expected pattern? Does speeding or slowing the pace change the emotions?
Magazine story - more description, images a bonus, page turns unnecessary, read aloud does not invite audience participation.
Picture book - description mostly left to the illustrator, images complete the story, page turns necessary for telling the story, great read aloud that often invites participation.