I could've sworn that "dumb" meant "mute", not deaf. Might be completely different in American, though...

Not all books use a different font, or some other way (maybe italics). Italics are generally used for thinking, that's nearly a rule. For sarcasm, though, it's either just given and the character indicates they were being sarcastic, or it is indicated that the character says it in a sarcastic tone.