bead proof

from The Century Dictionary.

• Of such a nature or quality that a crown of bubbles formed by shaking will stand for some time on the surface: said of spirituous liquors, and erroneously supposed to indicate strength.
• Of a certain standard of strength as ascertained by beads. See bead, n., 7.

from the GNU version of the Collaborative International Dictionary of English.

• 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.
• 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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, and later by more accurate meters.
• noun 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.