Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Wildly vacillating, light-hearted, prophetically apocalyptic
R.U.R. is not especially long as works of literature go, but it packs a lot of tonal variation into what space it's got. The result can be disorienting. One second, robots are a goofy gimmick, and you're chuckling at them like it's the first time you've heard an iPod talk to you. Then, the next moment, you're talking about Biblical doom and moaning about the dead bodies piled everywhere.
Happy?
The light-hearted tone is concentrated especially (though not solely) in the prologue. Much of the material in that act comes across as comedy, or even as slapstick. For example, the following exchange could come out of any modern romantic comedy:
Helena: And this is what I shouldn't tell anyone?
Domin: No one in the world.
Helena: It's a pity this is already in all the papers. (prologue.71-73)
The tone of the play there is totally jovial. Helena and Domin are both more than a little ridiculous. The whole thing is a joke.
Sad?
Then, in Act 3, you get passages like this:
Alquist: It's hopeless! These books no longer speak. They're as mute as everything else. They died, died, along with people! (3.4)
You could read that as comedy too, almost—it's so melodramatic and over the top it's kind of ridiculous. But the play doesn't seem to think it's ridiculous. Alquist is talking about the end of the world, after all. The attitude is serious. This is really supposed to fill you with angst and horror (yipes).
Who Knows?
At times, the vacillating (back and forth) tone is actively confusing. For instance, what are you supposed to make of this:
Nana: On your knees! The hour of judgment is upon us!
Hallemeier: Thunder, you're still alive!
Nana: Repent, you unbelievers! The end of the world is come! Pray! (2.355-2.357)
When Hallemeier says, "Thunder, you're still alive!"—is that a sitcom double-take, "ha ha, Nana is awful and Hallemeier was hoping she was dead"? Or, is the whole sequence supposed to express horror and melodramatic angst? If you were putting this play on, you could probably go either way. Perhaps Čapek himself was uncertain at times whether he was writing comedy or tragedy. Are robots a ridiculous idea or a terrifying one? The tone varies, it seems like, because Čapek isn't sure quite how he feels about his famous artificial creation.