Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Maybe it's just us, but we get a bad feeling when we read Walton's first letter to his sister:
I shall satiate my ardent curiosity with the sight of a part of the world never before visited, and may tread a land never before imprinted by the foot of man. These are my enticements, and they are sufficient to conquer all fear of danger or death and to induce me to commence this laborious voyage with the joy a child feels when he embarks in a little boat, with his holiday mates, on an expedition of discovery up his native river. (Letter 1.2)
You know how when you go see a horror movie, and it starts out with some really happy scene—say, some kids in a boat or skinny dipping in the ocean in the middle of the night—you just know that something bad is about to happen? That's how we read Walton's description of himself as a child in a little boat. Sure, maybe we're just projecting. After all, Frankenstein was written 150 years before Jaws. But Shelley was a savvy lady, and we're pretty sure she knew what she was doing.
Because there are so many different narrators, the tone shifts slightly throughout the text. Walton is a little more reserved and reportorial; Frankenstein is fatalistic ("Destiny was too potent" (2.12); the monster is enraged. But everyone seems to agree: bad things are going to happen.