Buy a bottle of Blue Nun, Schwarze Katze or Liebfraumilch. Then select a quality wine. Make sure “Riesling” appears on the label. Keep in mind that a quality Riesling will be at least twice as expensive as its sweet, bargain-basement counterpart. Also look for a Riesling that is at least 5 years old with any one of the following classifications: Kabinett, Spätlese or Auslese. Last but not least it is important for this exercise not to select a Riesling labeled trocken – dry.
Regardless of whether you like sweetie pies such as Blue Nun or Liebfraumilch, put your personal preferences aside. You’ll want your eyes, nose and palate to be objective when comparing the color, fragrance, and intensity and depth of flavor, respectively, of a quality Riesling to a cheap one.
Here’s a short list of producers of quality Riesling: Leitz, Lossen, Egon Müller, J.J. Prüm, von Schubert and Robert Weil.