Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Critical, but Sympathetic
Pasternak doesn't pull any punches when it comes to showing us people's failings. He doesn't, for example, let Zhivago off the hook for cheating on his wife with another (married) woman. But at the same time, he doesn't exactly condemn the guy to hell, either. Even though he's critical, Pasternak always seems to make us aware that people have their good points and their bad points.
For just one example, take a look at Zhivago's thoughts about cheating on Tonya: "In defense of her wounded pride he would have torn the offender to pieces with his own hands. And here that offender was he himself" (9.16.4). So... Zhivago would kill the dude who cheated on Tonya, but he recognizes that he is that dude. That's complicated stuff.
With this kind of tone, Pasternak is able to do two things at once. He's able to reveal the fact that Zhivago is a flawed man, and he's able to make us sympathize with the conflicting impulses that pull Zhivago in two different directions. This is a good skill to have when you write a book about something as controversial as Communism and the Russian Revolution, and Pasternak maintains this balance all the way through Doctor Zhivago.