You cannot know wine by the barrel.

A container does not reveal the quality of what it holds. The barrel may be visible, but the wine must be examined on its own terms.

More broadly, outward appearance is a poor basis for a firm judgment about what lies beneath it. Assess the thing itself before deciding its worth.

Use the proverb to caution against judging something solely by its exterior or presentation. It fits a situation where someone is drawing a firm conclusion before examining what actually matters.