One way to describe the criteria for optical activity: a molecule is optically active if it has a non super-imposable mirror image.
In the case of biphenyls you also have to consider the fact that many biphenyls have unusually high barriers for rotation around the central bond connecting the two aryl components. Note that not all biphenyl compounds are optically active: there must be restricted rotation around bond connecting the two aryl rings, and also they must have a special kind of substitution.