The full Mao quotation (from his speech The Orientation of the Youth Movement, held in Yan'an, May 4, 1939):

How should we judge whether a youth is a revolutionary? How can we tell? There can only be one criterion, namely, whether or not he is willing to integrate himself with the broad masses of workers and peasants and does so in practice. Mao Zedong

看一个青年是不是革命的, 拿什么做标准呢? 拿什么去辨 别他呢? 只有一个标准,这就 是看他愿意不愿意、并且实行 不实行和广大的工农群众结合 在一块。毛泽东