The lowest fineness of .750 (12 Loth) often used in Prussia, when tested, will also precipitate silver chromate (Ag2CrO4), sometimes barely visible with naked eye, but it`s there, proving silver alloy.
Another tip: after filing, the surface may not react as expected, leading to wrong conclusion it`s not a silver alloy. The test should be repeated; just wipe off using a cotton rag and apply another drop – red color will appear.
Sterling is not the problem, lower fineness, especially lower than .750 may be tricky. Do not underestimate low fineness – many beautiful items were made of .500 in e.g. Ottoman Empire, not well legislated and without any marks. Just look for precipitated red silver chromate (tiny red flakes within yellow- greenish liquid of the applied drop).
Again, it`s a qualitative test, more or less, although intensive, vivid red indicates high silver contents in the alloy.