Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Authoritative, Angry
Karl switches his tone up throughout the book, taking an abstract, theoretical tone in some sections—such as the first three of Chapter 1—and taking an angry one in others, such as most of Chapter 10, on the working day, where he mocks the capitalists and accuses them for their exploitative practices. Throughout, however, Marx has an authoritative attitude: he's the expert, you're reading his book telling you how it is, and your sad opinion is not asked for at all.
Marx's vibe is pretty much I know everything.
Take his assertion that the "wealth of societies in which the capitalist mode of production prevails appears as an 'immense collection of commodities'" (1.1.1), for example. Ol' Karl doesn't say to some extent or sometimes or maybe or you could make the argument that—nope, he doesn't hedge or hesitate at all. You can prove him wrong if you like, but he's not going to make your job any easier by conceding points.
Then there's the angry bit. Most of the time, Karl keeps his rage under check when he's discussing abstract theories, like the make-up of the commodity. But at other times, he does let loose. When he quotes the capitalist E. F. Sanderson saying employing men would cost more money than employing boys but he couldn't raise the price of steel to pay for that, Marx interrupts the quotation to stick in: "[H]ow wrong-headed these people are!" (10.4.10). This attitude pops up in various places throughout the text.