A standard sandwich tainted with human urine. Commonly used on white bread, its common tell signs are a yellow stain that is concentrated in one area. Unfortunately, there is no way to 100% tell if it is a standard sandwich, or a Pisswich.

The reason is, melted butter or margarine, has the same affects (on white bread). The only surefire way to insure that your sandwich is not a Pisswich, is to eat it, or use a DNA test on all the ingredients in said sandwich/Pisswich.
Common scenarios when one may find a Pisswich, is during sleep overs with friends, parties, weddings, cooking classes, trash cans, and funerals.

the art of remembering an arousing occurrence or some porn you watched while masturbating, often handy if you have no porn or your in the shower