To discern faults, though they be in fashion: Though Vice be clothed in cloth of gold, yet a good man will still know it. It is to no purpose for it to be apparelled in gold, it can never so well disguise it self but that it will be perceived to be of iron. It would cloak it self with the nobility of its Adherents, but it is never stript of its baseness, nor the misery of its slavery.

[Conocer los defectos, por más autorizados que estén. No desconozca la entereza el vicio, aunque se revista de brocado; corónase tal vez de oro, pero no por eso puede disimular el yerro. No pierde la esclavitud de su vileza aunque se desmienta con la nobleza del sujeto.]

Baltasar Gracián y Morales (1601-1658) Spanish Jesuit priest, writer, philosopher

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 186 (1647) [Flesher ed. (1685)]
    (Source)

(Source (Spanish)). Alternate translations:

Recognise Faults, however high placed. Integrity cannot mistake vice even when clothed in brocade or perchance crowned with gold, but will not be able to hide its character for all that. Slavery does not lose its vileness, however it vaunt the nobility of its lord and master.
[tr. Jacobs (1892)]

Know what is evil, however much worshiped it may be. Let the man of intelligence not fail to recognize it, even if clothed in brocade, or crowned with gold, because it cannot thereby hide its bane, -- slavery does not lose its infamy, however noble the master.
[tr. Fischer (1937)]

Know when something is a defect, even if it looks like the opposite. Honesty should be able to recognize vice even when it dresses in brocade. Sometimes it wears a crown of gold, but even then it cannot hide its iron. Slavery is just as vile when disguised by high position.
[tr. Maurer (1992)]