The first and probably the most important thing to remember is that, in order for you to know and even write the tone of a story, you must be able to familiarize the story and understand it in your own understanding.
Another important thing is to know the facts of the story, the characters' personality, their roles, the sequence of events and the like. Knowing each important role of the story helps shape the events and how they turned out. In setting the tone of the story, it mostly is reflected on the characters being portrayed. Is it hopeful? Cynical? Snarky? Playful?
Adding some parts like quotes from the story may help as concrete evidence on what the tone is, judging from the mood of the scene and everything as a whole.
Writing a tone of the story is kind of tedious for some but as long as you know how to do it, it wouldn't be a problem. Hope this helps!
