  We show that features of the dynamical spin susceptibility can unambiguously
distinguish between different pairing symmetries of the superconducting state
in iron-based superconductors. A magnetic resonance is a unique feature for the
extended $s_{x^2y^2}$-wave $\cos k_x\cos k_y$ pairing symmetry. It is present
in the pure superconducting (SC) state, but weakens in the mixed SC and
magnetically ordered state. We calculate the the RPA correction to the NMR spin
relaxation rate $1/T_1$ and the Knight shift in the above states and show a
good agreement with experimental results. Moreover, we argue that the energy
dispersion of the magnetic resonance along c axis observed in neutron
scattering experiments is also an indirect evidence supporting the $s_{x^2y^2}$
pairing symmetry.
