Nos livros

Nos livros, aço valiriano tem um notável efeito de "ondulação" no metal, provavelmente semelhante a uma lâmina de Damasco . Também foi descrito como tendo um aspecto "esfumaçado".

Além disso, foi afirmado por um ferreiro que

the two longswords forged from Ice (Oathkeeper & Widow's Wail) would not hold the crimson color requested by Tywin Lannister, but instead kept reverting to a smoky merger of crimson and silver.

Assim, podemos supor que o metal tem qualidades únicas causando esse efeito. Qualquer um que saiba o que procurar poderia, portanto, facilmente dizer o aço valiriano até mesmo do melhor aço não valiriano.

No programa de TV

As espadas valirianas no programa de TV parecem ter um brilho muito maior na lâmina. O polimento mais alto faz com que seja muito mais reflexivo, quase um efeito cromo. De fato, em várias cenas, Longclaw parece ter quase um efeito brilhante - muito sutil, mas ainda assim.

Isso é mais perceptível na 4ª temporada durante o

Raid on Craster's Keep. Both outside, as well as when inside fighting Karl, the blade has an almost imperceptible white glow to it.

Além disso, como apontado nos comentários, quando apresentado a ele, Jaime Lannister é capaz de reconhecer Oathkeeper como uma arma valiriana somente pela visão.

Aqui está a cena que eu menciono na tag spoiler:

Sim

O aço valiriano pode ser distinguido de outras lâminas com facilidade por suas propriedades. Isso dá o efeito de que as lâminas estão onduladas e que você poderia distinguir a lâmina do aço normal de senti-la.

The properties of Valyrian steel are well-known, and are the result of both folding iron many times to balance and remove impurities, and the use of spells—or at least arts we do not know—to give unnatural strength to the resulting steel. Those arts are now lost, though the smiths of Qohor claim to still know magics for reworking Valyrian steel without losing its strength or unsurpassed ability to hold an edge. The Valyrian steel blades that remain in the world might number in the thousands, but in the Seven Kingdoms there are only 227 such weapons according to Archmaester Thurgood's Inventories, some of which have since been lost or have disappeared from the annals of history.
_{The World of Ice and Fire, Ancient History: Valyria’s Children}

Mas e a sua aparência física? Bem, o aço valiriano tem um notável efeito de ondulação na dobra e geralmente tem uma cor cinza escuro.

Tyrion wondered where the metal for this one had come from. A few master armorers could rework old Valyrian steel, but the secrets of its making had been lost when the Doom came to old Valyria. "The colors are strange," he commented as he turned the blade in the sunlight. Most Valyrian steel was a grey so dark it looked almost black, as was true here as well. But blended into the folds was a red as deep as the grey. The two colors lapped over one another without ever touching, each ripple distinct, like waves of night and blood upon some steely shore. "How did you get this patterning? I've never seen anything like it."
_{A Storm of Swords, Tyrion IV}

Também temos alguns exemplos de caracteres que reconhecem uma lâmina de aço valiriana somente pela visão.

The pommel was a hunk of pale stone weighted with lead to balance the long blade. It had been carved into the likeness of a snarling wolf's head, with chips of garnet set into the eyes. The grip was virgin leather, soft and black, as yet unstained by sweat or blood. The blade itself was a good half foot longer than those Jon was used to, tapered to thrust as well as slash, with three fullers deeply incised in the metal. Where Ice was a true two-handed greatsword, this was a hand-and-a-halfer, sometimes named a "bastard sword." Yet the wolf sword actually seemed lighter than the blades he had wielded before. When Jon turned it sideways, he could see the ripples in the dark steel where the metal had been folded back on itself again and again. "This is Valyrian steel, my lord," he said wonderingly. His father had let him handle Ice often enough; he knew the look, the feel.
_{A Game of Thrones, Jon VIII}

The sword was much lighter than he had expected. As he turned it in his hand he saw why. Only one metal could be beaten so thin and still have strength enough to fight with, and there was no mistaking those** ripples**, the mark of steel that has been folded back on itself many thousands of times. "Valyrian steel?"
_{A Storm of Swords, Tyrion IV}