。
Look at the contents, not the container.
Literal
Container [object-を] see-[negative-ずに] contents [object-を] see-[imperative-よ].
A proverb. 見ずに ('without looking,' formal negative adverbial). 見よ is a literary/archaic imperative of 見る (modern: 見ろ). 中身 ('contents / insides') vs 容器 ('container / vessel'). The literary 見よ gives it a biblical or aphoristic tone.