  Within the standard electroweak model we point out that the $3\times 3$
matrix of quark mixing is characterized by three universal
(rephasing-invariant) quantities: one of them for $CP$ violation and the other
two for off-diagonal asymmetries. Unitarity of the quark mixing matrix can in
principle be tested through a variety of measurements which are irrelevant to
the existence of $CP$-violating effects.
