Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Provocative, Angry, Optimistic
Three words do a darn good job of describing Marx's attitude in the Manifesto. Provocative, angry, and optimistic. Works well for a manifesto, if you ask us.
Provocative
Karl isn't here to be your sweetheart. He's out to shock you into thinking differently, and that means he takes a deliberately provocative tone. Check out this example: "[Y]ou reproach us with intending to do away with your property. Precisely so; that is just what we intend" (Section2.28). He is addressing you—well, if you are an upper-class bourgeois owner of capital or something like that—and saying that he and his crew are gonna come take—er, liberate your stuff from you. Watch out.
Angry
You can feel the author's blood boiling in many passages. For instance: The bourgeoisie "has pitilessly torn asunder the motley feudal ties that bound man to his 'natural superiors,' and has left remaining no other nexus between man and man than naked self-interest, than callous 'cash payment.' It has drowned the most heavenly ecstasies of religious fervour, of chivalrous enthusiasm, of philistine sentimentalism, in the icy water of egotistical calculation [...] naked, shameless, direct, brutal exploitation" (Section1.14).
Or how about this?: "But don't wrangle with us so long as you apply, to our intended abolition of bourgeois property, the standard of your bourgeois notions of freedom, culture, law, &tc." (Section2.36).
Marx sees all this injustice and just feels flat-out mad about it. And guess what? It's kind of catching. Seeing the old guy get so worked out kind of makes you, the reader, start feeling those things, too. It's one of Marx's strategies for getting you on his side.
Optimistic
Even with all the anger and provocation, Marx still stays briskly optimistic, convinced history is on his and the workers' side. We mean, seriously, just take a look at the final lines of the manifesto: "The Communists disdain to conceal their views and aims. They openly declare that their ends can be attained only by the forcible overthrow of all existing social conditions. Let the ruling classes tremble at a Communistic revolution. The proletarians have nothing to lose but their chains. They have a world to win. WORKING MEN OF ALL COUNTRIES, UNITE!" (Section4.11-12).
Hey, we guess if you're trying to change the course of world history, it probably helps if you're pretty sure it's actually going to happen.