I'm sure in Westeros there are some incredibly well crafted steel swords, but could someone instantly spot a Valyrian steel sword over one of these?

Does Valyrian steel have any visual indications that it is Valyrian steel and not just a very well crafted steel sword?

I'm wondering, in canon, has anyone seen a sword and instantly recognised it as Valyrian steel.

For the purpose of this question, no one is allowed pick up or use the sword, so they cant feel it's weight, balance, sharpness, etc...

TLDR: Yes, Valyrian steel is visibly different from even the best steel.

In the Books

In the books, Valyrian steel has a noticeable "ripple" effect to the metal, probably similar to a Damascus blade. It's also been described as having a "smoky" look.

Also, it was stated by a blacksmith that

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.

Thus we can assume that the metal has unique qualities causing this effect. Anyone who knows what to look for could therefore easily tell Valyrian steel from even the finest non-Valyrian steel.

On the TV Show

The Valyrian swords in the TV show appear to have a much higher polish to the blade. The higher polish causes it to be much more reflective, almost a chrome effect. In fact, in several scenes, Longclaw appears to almost have a glowing effect to it - very subtle, but there nonetheless.

This is most noticeable in Season 4 during the

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

Also, as pointed out in the comments, when presented with it Jaime Lannister is able to recognize Oathkeeper as a Valyrian weapon by sight alone.

Removed video - no longer available

Yes

Valyrian steel can be told apart from other blades easily enough by it's properties alone. This gives the effect that the blades are rippled and that you could tell the blade apart from normal steel from feeling it.

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}

But what about it's physical appearance? Well Valyrian steel has a noticeable ripple effect from the folding and is usually quite a dark grey colour.

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}

We also have a few examples of characters recognising a Valyrian steel blade by sight alone.

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}