We report a spectroscopic second harmonic (SH) study of Si(001) surfaces under both ultrahigh vacuum and epitaxial growth conditions which includes in situ azimuthal rotation of the sample. The results show a strong influence of surface hydrogen termination and bulk doping concentration on the SH spectrum and azimuthal anisotropy. We propose a qualitative model in which a combination of bulk and surface electric-field induced SH polarizations, which augments the usual surface dipole and bulk quadrupole SH polarizations, are primarily responsible for the sensitivity to surface hydrogen and bulk doping. We also report an in situ real-time SH study of disilane adsorption onto and hydrogen desorption from Si(001) surfaces during epitaxial growth. The abilities to detect bulk doping concentration and monitor the growth rate in real time make SH spectroscopy promising as a growth sensor.