How can you tell real 925 argent sterling?
When a piece of silver has that stamp on it, it means that the maker has sent either the finished piece or the component parts to a hallmarking service who analyse the metal to make sure that it is genuine sterling before stamping each piece for you – or I believe larger-scale producers with trusted materials can buy their own stamp for a LOT of money. So if the piece was made in a place like the UK or the US, you’re probably pretty safe.
Having said that, forged stamps can be used by less trustworthy suppliers like a lot of those discount Chinese sellers so in that case it is worth being cautious. Calculating the density is complicated because you can’t compare it directly to the density of elemental silver, due to the other components of the alloy.
The best ways to be sure are to leave it to tarnish – this happens naturally over a few weeks, but you can speed it up either by keeping it somewhere warm and humid or by putting it in a ziplock back with a couple of boiled egg yolks. If the silver turns blue-black on the surface, you know it’s at least silver plated.
Beyond that, it’s difficult to confirm for certain without somehow damaging the item, so you might want to take it to your favourite local jeweller for confirmation. They might even be able to tell you just from looking at the piece. A well-trained eye can simply spot silver; it has a subtle but definitive shade compared to other silver metals like white gold, zinc-based alloys etc.