The nuclear spin system of a semiconductor can be cooled down to the microKelvin range of spin temperatures using optical pumping with subsequent adiabatic demagnetization. The efficiency of the nuclear spin cooling depends on the local field B_L , which is created by dipole - dipole interaction between nuclear spins. In this work we develop a method of experimental determination of the local field in semiconductor structures by using optical cooling and adiabatic demagnetization of the nuclear spin system, and apply it to measure B_L in n-GaAs.