  At proposed high energy linear e+e- colliders a large number of W bosons and
top quarks will be produced. We evaluate the potential precision to which the
decay branching ratios into the various quark species can be measured, implying
also the determination of the respective CKM matrix elements. Crucial is the
identification of the individual quark flavours, which can be achieved
independent of QCD models. For transitions involving up quarks the accuracy is
of the same order of magnitude as has been reached in hadron decays. We
estimate that for charm transitions a precision can be reached that is superior
to current and projected traditional kinds of measurements. The t->b
determination will be significantly improved, and for the first time a direct
measurement of the t->s transition can be made. In all cases such a
determination is complementary to the traditional way of extracting the CKM
matrix elements.
