Most men know to signify to a friend that a girl has a B.U.V. by standing in said friends line of vision behind the girls back and letting his tongue hang loosely out of the side of his mouth. The friend, now empowered with the knowledge can proceed with caution.
It is a lesser known fact that dogs are known as man's best friend because of their propensity to alert men of being in the vicinity of a B.U.V. using the same method of hanging their tongue loosely out of their mouth as described above. The uninformed and hippies would say that is silly and the dog is simply smiling. They would be wrong.