gold unlke most metals is also not magnetic so you could test it with a simple magnet. you can drag it accros a unglazed ceramic plate to see if there is a gold mark on it. you can test the density with a scale and a graduated cylinder to determine the valuim.
source: https://www.bullionbypost.co.uk/index/gold/how-to-tell-if-gold-is-real/
That consensus is already present precataclysm and where many currencies get there value do to the trust placed in the government that isseus them (which are now all gone) noble metals are seen as having innate value and i didn’t say that gold would be THE currency everyone would be using but it would be used as de defecto currency along side other high value items that are relatively easy to transport, store and don’t spoil (ammo and drugs) to make trading easier so you don’t have to drag around a lot of stuff if you want to buy something.
It would be worthless in trade since paper is so readelly available you can find it really aywhere in the game so no body in their right mind would trade for it if i can just find it lying around for free everywhere. this isn’t the case for gold so if i trade for gold i know for a fact that you can’t just find more of it laying around everywhere since it is so rare.
gold is still pretty rare both in real live (you could fit all the gold above ground in a cube 20,7m on each side) in game the quantities you would and do get from looting are very small and gold and silver are still far rarer than other things like ammo, guns, drugs or food but not so rare that they couldn’t be used as a regular medium of trade like purifier.
as for the central agreement on gold being valueable. Everyone from before the cataclysm would have learned that gold would be valueable this is something that everybody believes independent of what the government does or says so i do see the dollar losing all value given the government collapse but i don’t see this happening for gold since it’s precieved value relies on that everyone already believes it to be valueable and knows everyone els believes this as well. this doesn’t change with the cataclysm. you do make a fair point however that gold wouldn’t be used as a semi-currency directly after the cataclysm since it would take time for trade and practices around it to develop as the remnents of humanity form factions. however it would seem logical to me that after groups had formed and more large scale trade developed the need for some sort of goods/items that could function as semi-currensies would be needed for practical reasons and for convience (why bring a few kg’s of food with you if you are looking to trade if you can just take a few grams of drug with you with the same value) gold is a perfect candidate to fill that roll beside things like drugs and ammo.
see my previous answer for the last point