bead proof

 Among distillers, a certain degree of strength in alcoholic liquor, as formerly ascertained by the floating or sinking of glass globules of different specific gravities thrown into it; now ascertained by more accurate meters.  [1913 Webster]

 A degree of strength in alcoholic liquor as shown by beads or small bubbles remaining on its surface, or at the side of the glass, when shaken.  [1913 Webster]