In that you may be looking at a bar that has been created from melting silver plated objects, testing is critical.
Chris and others have mentioned the use of Schwerter's solution, which reacts blood red with silver. Do not rely on Schwerter's, alone. I suggest you test with both nitric and Schwerter's. A drop of Schwerter's on sterling should react by displaying red, then it should slowly darken with time. The percentage of copper is low, so silver predominates. By contrast, if you test sterling with a drop of nitric, the base color will turn a creamy white, and will yield silver chloride (white cottage cheese like substance when in volume) upon the addition of either HCl or table salt. If the nitric test yields a bold blue color rapidly, it's a pretty good sign that what has been melted most likely is not sterling, but probably silver plating. The Schwerter's test in such a situation is likely to yield a more brown reaction than red, and if the silver content is low enough, it will be green.
I agree that you will be unable to determine a percentage of any given element (silver, copper, nickel), but by conducting these tests you should be able to make a determination if you're being snookered, or not. It is very common for folks to sell white alloys that bear no silver, yet it is represented to be silver.
Harold