The Vinometer is designed to test the alcohol level of finished wines. While reasonable accurate for dry wines, it will not test the alcohol level of sweet wines, unfinished wines, beer, or other beverages. It works by measuring the surface tension of a liquid, and equating it to known standards of wine.
To use, pour a few drops of wine into the cup on top of the vinometer. Allow the wine to run out of the hole on the bottom, and then turn it upside down for the reading. Read where the bottom of the wine column mets the scale, and that is your alcohol percentage by volume.