  There are many charmless B decay pairs whose amplitudes are related by U spin
(d <--> s) or flavor SU(3). The theoretical uncertainty in any analysis
involving such pairs must take into account U-spin/SU(3) breaking. In the past,
such considerations generally used theoretical input, but we show that this can
be experimentally measured. We present lists of two- and three-body decay pairs
from which the size of the breaking can be obtained. We detail the values of
U-spin/SU(3) breaking given by the present experimental data. One pair -- Bd ->
pi+ pi- and Bd -> pi- K+ -- exhibits large nonfactorizable breaking. We present
other signals of SU(3) breaking in two- and three-body decays, and discuss
further tests for nonfactorizable effects. Finally, we also point out that the
pure-penguin decay Bs -> Kbar0 Kbar0 K0 is intriguing because it can be used to
cleanly probe the Bs-Bsbar mixing phase.
