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A tree is known by its fruit.
Literal
Tree [topic-は] fruit [object-を] see-[if-ば] is-understood.
A biblical-flavoured proverb (cf. Matthew 7:16). The structure is a conditional — 'if you look at the fruit, you understand (the tree).' The ば-conditional is a natural fit for aphorisms and general truths. わかる, as always, takes the thing being understood as its subject — here implicitly 木 itself.