Some modern models of neutron star evolution predict that initially large magnetic fields rapidly decay down to some saturation value ∼ few × 10 13 G and weaker magnetic fields do not decay significantly (Pons et al., 2009). It is difficult to check the predictions of this model for initially highly magnetized objects on the time scale of a few million years. We propose to use Be/X-ray binaries for this purpose. We apply several methods to estimate magnetic fields of neutron stars in these accreting systems using the data obtained by the RXTE satellite (Galache et al., 2008). Only using the most modern approach for estimating the magnetic field strengths of long period NSs as proposed by Shakura et al. (2011) we are able to obtain a field distribution compatible with predictions of the theoretical model of field decay of Pons et al. (2009).