  We present a universal approach for determining the spontaneous polarization
Psp of a wurtzite semiconductor from the emission energies of excitons bound to
the different types of stacking faults in these crystals. Employing
micro-photoluminescence and cathodoluminescence spectroscopy, we observe
emission lines from the intrinsic and extrinsic stacking faults in strain-free
GaN micro-crystals. By treating the polarization sheet charges associated with
these stacking faults as a plate capacitor, Psp can be obtained from the
observed transition energies with no additional assumptions. Self-consistent
Poisson-Schroedinger calculations, aided by the microscopic electrostatic
potential computed using density-functional theory, lead to nearly identical
values for Psp. Our recommended value for Psp of GaN is -0.022+/-0.007 C/m^{2}.
