How is the theme of outward show seen in Bassanio’s speech in act 3, scene 2 of The Merchant of Venice?

As he gets ready to take the casket test, Bassanio makes a little speech. And in that speech, he shows an understanding of the principle behind the test devised by Portia's late father. The old man wanted to make sure that those gold-diggers who'd beat a path to his daughter's door would be so dazzled by the riches on offer that they'd make the wrong choice of casket, thus leaving the field open to a man who loved Portia for herself.
Bassanio is that man. And in his little speech, he shows how much he understands that all that glitters isn't gold:

So may the outward shows be least themselves. The world is still deceived with ornament. (act 3, scene 2)

The world may be deceived with ornament, but not Bassanio. He wisely plumps for the "humble" lead casket, inside of which is a picture of Portia. As he makes his choice, he reflects on how the lead casket threatens more than it promises, but nonetheless draws him towards it, its paleness speaking more eloquently than any words:

But thou, thou meagre lead, Which rather threaten’st than dost promise aught, Thy paleness moves me more than eloquence, And here choose I. Joy be the consequence! (act 3, scene 2)