The heat effected zone will have a different chrystal structure than the base metal.
I look for: Waviness in the hook area, regardless of the level of polishing. Smoking will reveal lengthwise low spots that can't be explained by reaming. No one files perfectly.
Another lead is if you smoke the hook, the new scratches on the side of the lump will not be on the same radius as the older scratches on the lump.
HAZ shimmer is a giveaway. No hook scratches another. (Where'd they go? The pin has scratches, where's hook's matching scratches?)
The "wetting area" between the filler rod, the haz and the base metal is like a dull silver ribbon. Neither fish nor fowl (It's a different alloy), and doesn't belong there if the hook was never welded.
But all that is about a yes or no decision. If there is evidence of repair, then what? Negotiate a lower price? Question veracity of seller?
Peace of mind?
Even in the highest grade collectibles, there is provision for legitimate skilled, or workman-like repair.
Maybe Steve Bertram will photograph a welded hook, showing the heat effected zone blemishes.
