This one is always a dilemma; recently a relative told me how he was overseas and wanted to buy a chain for his fiance. He was looking in a silver shop but didn’t know how to tell if a chain was real silver or not, so he asked how I know.
It’s hard to be 100% sure, but some things to look for are if the piece says .925 or S or SS. This usually seals the deal. The problem with some shady sellers is that that other 7.5% of a sterling piece can be whatever metal they want. Most reputable dealers avoid lead or nickel, but is not always guaranteed. Nickel is what most people are allergic to if they have a bad reaction to base metals.
If you find a piece that is not labeled it doesn’t mean that it isn’t necessarily sterling. Things you can look out for are the feel and look of the piece. If it feels lighter than you think it should then it is probably fake. Additionally, when people try to pass off sterling they will sometimes try to make it SUPER shiny… and by SUPER shiny I mean painfully bright. Sterling can be a very shiny metal, but fake sterling just has this look about it.
Magnets are also a handy tool. Sterling is not magnetic, so if you hold a powerful magnet over them and they are attracted to it, then it is fake.
A lot of the techniques I use are based on context, otherwise your only other options are acid tests, which can damage the piece and is hard to carry around, or x-ray the piece, equally hard to carry around.