We found bulk n-GaAs layers grown by liquid phase epitaxy to be irregularly stressed. Deformation created by this stress causes a small but detectable quadrupole splitting of Zeeman nuclear energy levels. In our work we detected a very weak quadrupole splitting for 69Ga and 75As isotopes in bulk n-GaAs and obtained the value of the sample deformation. To this end, we used a new method that we call warm-up spectroscopy of nuclear spins in weak external magnetic fields.