You can identify them by their fruit, that is, by the way they act. Can you pick grapes from thornbushes, or figs from thistles?
Notice the word here is “identify”? So often we can catch ourselves “judging” others rather than looking at the fruit they produce. The true character of a person cannot be observed in a moment. We should always strive not to judge but rely on the discernment of the Holy Spirit.