  Ferromagnetic materials with exchange fields E_ex smaller or of the order of
the superconducting gap Delta are important for applications of corresponding
(s-wave) superconductor/ ferromagnet/ superconductor (SFS) junctions. Presently
such materials are not known but there are several proposals how to create
them. Small exchange fields are in principle difficult to detect. Based on our
results we propose reliable detection methods of such small E_ex. For exchange
fields smaller than the superconducting gap the subgap differential conductance
of the normal metal - ferromagnet - insulator - superconductor (NFIS) junction
shows a peak at the voltage bias equal to the exchange field of the
ferromagnetic layer, eV=E_ex. Thus measuring the subgap conductance one can
reliably determine small E_ex < Delta. In the opposite case E_ex > Delta one
can determine the exchange field in scanning tunneling microscopy (STM)
experiment. The density of states of the FS bilayer measured at the outer
border of the ferromagnet shows a peak at the energy equal to the exchange
field, E=E_ex. This peak can be only visible for small enough exchange fields
of the order of few Delta.
