The Collaborative International Dictionary of English v.0.48:

Bead proof \Bead" proof`\
   1. 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]

   2. 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]