What clues(besides willy saying his sons look like Adonises) show Willy believes in appearance?
We see several instances where Willy seems quite fixated in physical appearances and also in specific "visual stamps" that cause a sense of physical awe. Aside from his sons, we know how Willy thinks of himself as a person of high physical gifts who has the ability to "move crowds" and cause reactions.
…because the man who makes an appearance in the business world, the man who creates personal interest, is the man who gets ahead […] You take me, for instance. I never have to wait in line to see a buyer. “Willy Loman is here!” That’s all they have to know, and I go right through.
Another obvious factor in Willy's weakness for appearances is the fact that he takes in a mistress. "The Woman" is apparently an object to boost Willy's low ego. We know this because of the way that he asks her questions about himself, and why she would pick him as a lover
Willy: And you like me.
The woman: Sure. Because you're so sweet. And such a kidder!
Moreover, we see that Willy, (Act I) has this ridiculous idea that the reason why people do not "take" to him is, in part, because he may be fat. This is a similar trait that we will later find in his son, Happy, who is also fixated in showing off his weight loss to the family. Here, you can tell that Willy's ego is skin deep and so superficial that he cannot differentiate person from personality.
WILLY: I’m fat. I’m very—foolish to look at, Linda. I didn’t tell you, but Christmas time I happened to be calling on F.H. Stewarts, and a salesman I know, as I was going in to see the buyer I hear him say something about—walrus. And I—I cracked him right across the face. I won’t take that. I simply will not take that. But they do laugh at me. I know that.
WILLY: I gotta overcome it. I know I gotta overcome it. I’m not dressing to advantage, maybe.
One last instance is the fact that Willy looks down on Bernard, Charley's son, during his younger years. Although Bernard is intelligent and a very good friend to Biff, Willy totally condescends him simply because Bernard may look nerdy and is not a popular jock like Biff. Willy even warns Biff to be like Bernard instead of supporting him to study like Bernard does.
Hey, looka, Bernard! What are you looking so anaemic for!?
This is a quote from when Bernard insists on Biff's studying with him for the Regents test. Willy sees this as an insult to Biff, who (in his mind) only for his good looks and talent in football will have a sure entrance to the University of Virginia.
All of these instances have one thing in common: Willy comes out of each of them as the biggest loser. His kids grow up lost, Biff never enters the U of Virginia because he does flunk Math after all, then Biff discovers his father's mistress and all goes downhill afterward. Even more sad, the"anaemic" Bernard ends up a successful attorney in D.C.
Willy waits too late in life to realize that none of the things that he cares about actually matter. It is too late, however, to turn the clock back and change things over.