I have used the testing method, with an acid changing colour if it is indeed silver. However, if you are dealing with a silver plated item, it is a lot more difficult. You can check with the acid on parts that have seen the most wear, but it is not conclusive. You can make a small nick in the piece, and test it there. I have never attempted to distinguish between 925 and lower grades of silver. I have seen professionals do it, but it takes a lot of practice to distinguish the shade. To me it just gives me a pretty good indication: is it silver, or something else?

Generally I also like to have a close look at the quality of the work. The higher the quality and the finish, the more likely it is actual silver.