What element of a written work best determines if the work is sarcastic or not

Read the passage below and answer the question that follows. ‘you make me feel uncivilized, daisy,’ i confessed on my second glass of corky but rather impressive claret. ‘can’t you talk about crops or something? ’ i meant nothing in particular by this remark but it was taken up in an unexpected way. ‘civilization’s going to pieces,’ broke out tom violently. ‘i’ve gotten to be a terrible pessimist about things. have you read ‘the rise of the coloured empires’ by this man goddard? ’ ‘why, no,’ i answered, rather surprised by his tone. ‘well, it’s a fine book, and everybody ought to read it. the idea is if we don’t look out the white race will be—will be utterly submerged. it’s all scientific stuff; it’s been proved.’ in this passage, tom’s ideas about race relations come off as uncivilized. what literary device is fitzgerald using here?