  We propose an efficient method to identify the degree of frustration in
quasi-2D antiferromagnets described by the J_1-J_2 Heisenberg model. The
frustration ratio J_2/J_1 is usually obtained from analysis of susceptibility,
specific heat and saturation field. We show that the non-monotonic field
dependence of the staggered moment caused by the suppression of quantum
fluctuations in a field depends strongly on the frustration ratio. This gives a
powerful criterion to determine J_2/J_1 using a combination of exact
diagonalization (ED) method for finite clusters and spin wave analysis. We
apply this method to the quasi-2D compound Cu(pz)_2(ClO_4)_2 and show that it
leads to an intermediate ratio J_2/J_1 ~ 0.2 for the frustration. We also
explain the observed anomalous increase of transition temperature in applied
fields as an effect of reduced quantum fluctuations.
